Chapter XI: Literature and History (30)

Central chamber a little larger than both cupolas. From the surface arise twelve strong conical radial spines, four on the poles of the major and the minor axis of the shell, lying in the lateral plane; eight others lying in two diagonal planes, as prolongations of the eight wing-beams, which connect the _Tetrapyle_-shaped medullary shell with the two ring-like constrictions of the cortical shell. Pores of the latter irregular, roundish, twice to four times as broad as the bars; sixteen to eighteen in the basal semicircle of one cupola.

_Dimensions._--Major axis of the shell 0.15, minor 0.11; pores 0.004 to 0.008, bars 0.002.

_Habitat._--North Pacific, Station 256, surface.

5. _Amphitholus acanthometra_, n. sp. (Pl. 10, figs. 5, 6).

Central chamber about twice as broad and as high as both cupolas. On the surface quite symmetrically distributed twenty long and strong cylindrical radial spines; eight in the sagittal plane or in the meridian plane of the central chamber (four opposite in pairs in the principal and the sagittal axes, four others in the middle between the principal and the sagittal spines); four in the horizontal transverse plane, on both sides of the poles of the major or lateral axis; and eight in diagonal planes (corresponding to the eight wing-spines of _Tetrapyle octacantha_). In the centre of the central chamber is a distinct trizonal medullary shell like that of _Tetrapyle_, with two vertical columella beams. Pores of the cortical shell subregular, circular, with elevated hexagonal frames, about four times as broad as the bars; five to seven in the semicircle of one cupola. (This remarkable species differs from _Tholartus vicenus_ in the possession of a medullary shell, from _Amphitholonium acanthometra_ by the simple cortical shell. Compare these species).

_Dimensions._--Major axis of the shell 0.14 to 0.16, minor axis 0.10 to 0.12; pores 0.015, bars 0.004; major axis of the medullary shell 0.04 to 0.05, minor (horizontal) axis 0.02 to 0.03; length of the twenty radial spines 0.18 to 0.24, basal breadth 0.01.

_Habitat._--South Pacific, Station 300, depth 1375 fathoms.

{668}6. _Amphitholus armatus_, n. sp.

Central chamber twice as broad, but of the same height, as the flat vaulted cupolas. Surface armed with thirty to forty very large three-sided prismatic spines, longer than the major axis of the shell. Pores irregular, roundish, twice to five times as broad as the bars; eight to nine on the basal semicircle of one cupola.

_Dimensions._--Major axis of the shell 0.15, minor axis 0.1; pores 0.006 to 0.015, bars 0.003; length of the spines 0.2 to 0.25, breadth 0.01.

_Habitat._--North Atlantic, Station 353, surface.

7. _Amphitholus polyacanthus_, n. sp.

Central chamber of the same breadth and height as both cupolas. Surface armed with very numerous (sixty to eighty) strong conical spines, about as long as the minor axis of the shell. Pores subregular, circular, twice as broad as the bars; sixteen to eighteen in the basal semicircle of one cupola.

_Dimensions._--Major axis of the shell 0.13, minor 0.08; pores 0.006, bars 0.003; length of the radial spines 0.09, breadth 0.006.

_Habitat._--South Atlantic, Station 332, depth 2200 fathoms.

8. _Amphitholus panicium_, n. sp. (Pl. 10, fig. 4).

Central chamber ellipsoidal, nearly twice as broad, but of the same height, as both cupolas. Whole surface thorny; the largest radial thorns about as long as the medullary shell. Pores irregular, roundish, twice to four times as broad as the bars. On the base of each cupola a circle of ten to twelve larger square pores, separated by radial beams, which are prolonged into stouter free spines. This remarkable formation of the cupolas, very rare in this family, recalls the characteristic formation of the distal chambers of the Panartida and Zygartida (_e.g._, Pl. 40, figs. 4, 8, &c.), where it is very common.

_Dimensions._--Major axis of the shell 0.15, minor 0.1; pores 0.005 to 0.012, bars 0.003; large square pores 0.02; axes of the medullary shell 0.03 and 0.02.

_Habitat._--Indian Ocean, Madagascar, Rabbe, surface.

Genus 292. _Amphitholonium_,[331] n. gen.

_Definition._--#Tholonida# with double cortical shell (with external veil), with two hemispherical cupolas, opposite on the poles of one axis; central chamber (between them) _Larnacilla_-shaped, with medullary shell.

The genus _Amphitholonium_ differs from the nearly allied _Amphitholus_ (probably its ancestral form) only in the duplication of the cortical shell; the outer has the {669}same three-jointed form as the inner, and is connected with it by a variable number of radial beams. From the similar _Tholodes_ it differs in the possession of a medullary shell.

1. _Amphitholonium tricolonium_, n. sp. (Pl. 10, fig. 7).

Outer cortical shell smooth, of the same three-jointed form as the inner, at an equal distance from it throughout the whole circumference; central chamber higher-vaulted than both cupolas. Network of the outer shell delicate, with subregular, circular pores. Pores of the thick-walled inner shell subregular, circular, with hexagonal frames, three times as broad as the bars; eight to ten in the basal semicircle of one cupola.

_Dimensions._--Major axis of the outer cortical shell 0.2, minor axis 0.15; major axis of the inner cortical shell 0.16, minor axis 0.11; pores 0.01, bars 0.0035.

_Habitat._--South Pacific, Station 302, depth 1450 fathoms.

2. _Amphitholonium octostylium_, n. sp.

Outer cortical shell lentelliptical, smooth, not articulated, at a varying distance from the three-jointed inner shell; central chamber of the latter twice as high as both hemispherical cupolas. Network of the outer shell very thin, cob-web like. Pores of the inner shell subregular, roundish, twice as broad as the bars; six to seven in the semicircle of one cupola. Eight thin and long, bristle-shaped, radial spines, opposite in pairs in two crossed diagonal planes.

_Dimensions._--Major axis of the outer cortical shell 0.16, minor 0.12; major axis of the inner cortical shell 0.12, minor 0.09; pores 0.01, bars 0.005.

_Habitat._--Pacific, central area, Station 273, depth 2350 fathoms.

3. _Amphitholonium acanthonium_, n. sp.

Outer cortical shell of the same three-jointed form as the inner, at a uniform distance from it; central chamber higher vaulted than both cupolas. Network of the outer shell irregular, delicate, of the inner regular, strong, with circular, hexagonally framed pores, three times as broad as the bars; eight to nine in the semicircle of one cupola. On the surface, quite symmetrically disposed, twenty long and strong, cylindrical, radial spines; eight in the sagittal plane, four in the transverse plane; eight in two diagonal planes between the former and the latter. The remarkable geometric disposition of the twenty spines is in this species quite the same as in _Amphitholus acanthometra_. It differs from this nearly allied species in the double cortical shell.

_Dimensions._--Major axis of the outer cortical shell 0.18, minor 0.14; major axis of the inner cortical shell 0.14, minor 0.1; pores 0.012, bars 0.004.

_Habitat._--South Pacific, Station 295, surface.

{670}Subfamily 2. STAUROTHOLIDA, Haeckel.

_Definition._--#Tholonida# with diaxial growth; cupolas crossed in pairs, opposite on the poles of two axes, perpendicular one to another. (Shell commonly five-chambered, with four domes, cross-wise surrounding the central chamber.)

Genus 293. _Tholostaurus_,[332] n. gen.

_Definition._--#Tholonida# with simple cortical shell (without external veil), composed of four hemispherical cupolas in cross-form, opposite in pairs on the poles of two axes perpendicular one to another; central chamber simple (without medullary shell).

The genus _Tholostaurus_ is the most simple form of the Staurotholida, or the Tholonida with four crossed hemispherical cupolas, lying on the poles of two axes perpendicular one to another. The central chamber communicates by four wide openings with the four domes, and contains no medullary shell. _Tholostaurus_ may originate either from _Tholartus_ by apposition of two opposite cupolas between the first pair, or from _Staurotholus_ by loss of the medullary shell.

Subgenus 1. _Tholostaurantha_, Haeckel.

_Definition._--Surface of the shell smooth or rough, without radial spines.

1. _Tholostaurus quadrigatus_, n. sp.

All four cupolas nearly of the same size and form, subregular. Surface smooth, without radial spines. Pores subregular, circular, four times as broad as the bars; eight to ten pores in the basal semicircle of one cupola.

_Dimensions._--Diameter of the shell 0.12; pores 0.008, bars 0.002.

_Habitat._--Pacific, central area, Station 272, surface.

2. _Tholostaurus cruciformis_, n. sp.

Two opposite cupolas larger than the two others. Surface rough, without radial spines. Pores regular, circular, hexagonally framed, twice as broad as the bars; twelve to fourteen in the basal semicircle of one cupola.

_Dimensions._--Major axis of the shell 0.16, minor axis 0.12; pores 0.006, bars 0.003.

_Habitat._--Pacific, central area, Station 265, depth 2900 fathoms.

{671}Subgenus 2. _Tholostauroma_, Haeckel.

_Definition._--Surface of the shell with radial spines or thorns.

3. _Tholostaurus tetrabelonis_, n. sp.

All four cupolas nearly of the same size and form, subregular. Pores regular, circular, hexagonally framed, twice as broad as the bars; twelve to fourteen in the semicircle of one cupola. Four long and stout conical radial spines, two vertical on the poles of the principal axis, two horizontal on the poles of the transverse axis.

_Dimensions._--Major axis of the shell 0.15, minor 0.13; pores 0.006, bars 0.003.

_Habitat._--South Pacific, Station 288, surface.

4. _Tholostaurus octobelonis_, n. sp.

Two opposite cupolas larger than the two others. Pores subregular, circular, three times as broad as the bars; ten to twelve in the semicircle of one cupola. Eight long and thin, needle-shaped, radial spines, in pairs crossed in two diagonal planes.

_Dimensions._--Major axis of the shell 0.16, minor 0.12; pores 0.01, bars 0.003.

_Habitat._--South Pacific, Station 291, surface.

5. _Tholostaurus dodecabelos_, n. sp.

Two opposite cupolas larger than the two others. Pores irregular, roundish, twice to three times as broad as the bars; six to eight in the semicircle of one cupola. Twelve thin and long radial spines, four on the poles of the two crossed axes (principal and lateral), eight crossed in two diagonal planes.

_Dimensions._--Major axis of the shell 0.14, minor 0.11; pores 0.006 to 0.01, bars 0.003.

_Habitat._--North Pacific, Station 244, depth 2900 fathoms.

6. _Tholostaurus polybelonis_, n. sp.

Two opposite cupolas larger than the two others. Pores circular, hexagonally framed, of the same breadth as the bars; ten to twelve in the semicircle of one cupola. Numerous (twenty to thirty or more) thin, bristle-like, radial spines, about as long as the radius of the shell.

_Dimensions._--Major axis of the shell 0.15, minor axis 0.12; pores and bars 0.007.

_Habitat._--Indian Ocean, Zanzibar, Pullen, depth 2200 fathoms.

Genus 294. _Tholoma_,[333] n. gen.

_Definition._--#Tholonida# with double cortical shell (with external veil), composed of four hemispherical cupolas in cross-form, opposite in pairs on the poles of two axes perpendicular one to another; central chamber simple (without medullary shell).

{672}The genus _Tholoma_ (Pl. 10, figs. 10, 13) differs from the preceding _Tholostaurus_ (its probable ancestral form) only in the duplication of the shell. The outer (secondary) shell has the same cross-form as the inner (primary) shell. In the two observed species the growth seems to be different, in the first species all four cupolas of each cross being of the same size, form, and age; in the second two opposite cupolas, larger and apparently older than the other two. _Tholoma_ is possibly the offspring of _Staurotholonium_, from which it may have been produced by loss of the medullary shell.

Subgenus 1. _Tholomantha_, Haeckel.

_Definition._--Surface of the shell smooth, without radial spines.

1. _Tholoma quadrigeminum_, n. sp. (Pl. 10, fig. 10).

All four chambers in each cortical shell nearly equal, subregular. Surface smooth, without radial spines. Structure of the network in both shells similar. Pores regular, circular, twice as broad as the bars; six to eight in the basal semicircle of one inner, ten to twelve in the semicircle of one outer cupola.

_Dimensions._--Diameter of the outer shell 0.14, of the inner 0.09; pores 0.006, bars 0.003.

_Habitat._--South Pacific, Station 302, surface.

Subgenus 2. _Tholomura_, Haeckel.

_Definition._--Surface of the shell with radial spines.

2. _Tholoma metallasson_,[334] n. sp. (Pl. 10, fig. 13).

Two opposite chambers in each cortical shell larger than the two others; the larger chambers of the inner shell corresponding to the smaller chambers of the outer shell. Network in both shells of the same structure, regular, with circular, hexagonally framed pores of the same breadth as the bars; eight to ten in the semicircle of an inner, fourteen to sixteen in the semicircle of an outer cupola. Numerous (twenty to thirty or more) thin, cylindrical, bristle-shaped, long radial spines, symmetrically disposed.

_Dimensions._--Major axis of the outer shell 0.26, minor axis 0.2; major axis of the inner shell 0.16, minor axis 0.13; pores and bars 0.006.

_Habitat._--Pacific, central area, Station 265, depth 2900 fathoms.

Genus 295. _Staurotholus_,[335] n. gen.

_Definition._--#Tholonida# with simple cortical shell (without external veil), composed of four hemispherical cupolas in cross-form, opposite in pairs on the poles of {673}two axes perpendicular one to another; central chamber _Larnacilla_-shaped (with enclosed medullary shell).

The genus _Staurotholus_ differs from _Tholostaurus_ in the possession of a medullary shell in the central chamber, and may be derived from this genus by its production. But it may also be derived from _Amphitholus_ by apposition of two secondary opposite cupolas between the two primary cupolas. The symmetrical position of an increasing number of radial spines in the different species is remarkable (resembling _Tholostaurus_ as well as _Amphitholus_).

Subgenus 1. _Staurotholissa_, Haeckel.

_Definition._--Surface of the shell smooth or rough, without radial spines.

1. _Staurotholus quadratus_, n. sp.

Surface of the cortical shell smooth. All four cupolas nearly of the same size and form; therefore principal and lateral axes equal. Pores regular, circular, with hexagonal frames, twice as broad as the bars; eight to ten on the basal semicircle of one cupola. Medullary shell square.

_Dimensions._--Diameter of the cortical shell 0.15; pores 0.01, bars 0.005.

_Habitat._--South Atlantic, Station 323, depth 1900 fathoms.

2. _Staurotholus cruciatus_, n. sp.

Surface of the cortical shell rough. The two principal cupolas somewhat larger than the two lateral cupolas; therefore the longitudinal axis longer than the transverse. Pores irregular, roundish, twice to three times as broad as the bars; twelve to sixteen in the semicircle of one cupola.

_Dimensions._--Major axis of the shell 0.16, minor 0.13; pores 0.005 to 0.009, bars 0.003.

_Habitat._--Pacific, central area, Station 266, depth 2750 fathoms.

Subgenus 2. _Staurotholura_, Haeckel.

_Definition._--Surface of the shell with radial spines.

3. _Staurotholus tetrastylus_, n. sp. (Pl. 10, fig. 8).

The two principal cupolas smaller than the two lateral cupolas. Pores subregular, circular, three times as broad as the bars; six to eight in the semicircle of one cupola. Medullary shell elliptical. On the surface four long cylindrical radial spines; two in the principal and two in the lateral axis.

_Dimensions._--Major axis of the cortical shell 0.15, minor 0.12; pores 0.012, bars 0.004.

_Habitat._--South Pacific, Station 166, surface.

{674}4. _Staurotholus octostylus_, n. sp.

The two lateral cupolas larger than the two principal cupolas. Pores subregular, circular, twice as broad as the bars; ten to twelve in the semicircle of one cupola. Medullary shell hexagonal, connected with each ring-like constriction by eight strong cylindrical radial spines (longer than the whole shell), lying in two crossed meridian planes, and corresponding to the eight diagonal wing-spines of _Tetrapyle octacantha_.

_Dimensions._--Major axis of the cortical shell 0.12, minor 0.1; pores 0.008, bars 0.004.

_Habitat._--Pacific, central area, Station 274, depth 2750 fathoms.

5. _Staurotholus decastylus_, n. sp.

The two principal cupolas twice as broad and high as the two lateral cupolas. Pores subregular, circular, four times as broad as the bars; six to eight in the semicircle of one cupola. Ten long and thin radial spines with thickened conical bases, about as long as the major axis of the shell, by inner prolongations connected with the lentelliptical medullary shell; two polar spines in the principal axis, eight wing-spines in two crossed diagonal planes.

_Dimensions._--Major axis 0.12, minor 0.1; pores 0.012, bars 0.003.

_Habitat._--Western Tropical Pacific; Station 224, depth 1850 fathoms.

6. _Staurotholus dodecastylus_, n. sp. (Pl. 10, fig. 9).

The two lateral cupolas larger than the two principal cupolas. Pores subregular, circular, three times as broad as the bars; five to seven in the semicircle of one cupola. Twelve strong cylindrical radial spines, four on the poles of the two larger axes (two principal and two lateral), eight wing-spines in two crossed diagonal planes. Medullary shell hexagonal.

_Dimensions._--Major (lateral) axis 0.11, minor (principal) 0.09; pores 0.009, bars 0.003.

_Habitat._--Tropical Atlantic, Station 348, surface.

7. _Staurotholus polystylus_, n. sp.

The two principal cupolas larger than the two lateral cupolas. Pores irregular, roundish, twice to three times as broad as the bars; ten to twelve in the semicircle of one cupola. On the surface numerous thin bristle-shaped spines (twenty to thirty or more).

_Dimensions._--Major axis 0.14, minor 0.11; pores 0.006 to 0.01, bars 0.003.

_Habitat._--North Pacific, Station 231, surface.

Genus 296. _Staurotholonium_,[336] n. gen.

_Definition._--#Tholonida# with double cortical shell (with external veil), composed of four hemispherical cupolas in cross-form, opposite in pairs on the poles of two axes perpendicular one to another; central chamber _Larnacilla_-shaped (with medullary shell).

{675}The genus _Staurotholonium_ differs from the nearly allied _Staurotholus_ (its probable ancestral form) only in the duplication of the cortical shell. The outer shell commonly repeats the cross-form of the inner, with four corresponding cupolas; but sometimes the four cupolas of the outer shell alternate in size and form with those of the inner, or the outer shell forms a simple lenticular envelope around the inner. From the similar _Tholoma_, _Staurotholonium_ differs in the possession of a medullary shell in the central chamber.

Subgenus 1. _Staurotholodes_, Haeckel.

_Definition._--Surface of the outer cortical shell smooth or rough, without radial spines.

1. _Staurotholonium biquadratum_, n. sp.

Outer cortical shell smooth, of the same regular crucial form as the inner, equidistant from it everywhere. Form and structure of both cortical shells nearly the same, but the outer about twice as large as the inner; both connected only by eight diagonal beams. All four cupolas of each cortical shell regular, of the same size. Pores regular, circular, twice as broad as the bars; six to eight in the basal semicircle of each cupola. Medullary shell square. (Similar to _Staurotholus quadratus_, but differs by the double cortical shell.)

_Dimensions._--Diameter of the outer cortical shell 0.16, of the inner 0.08, of the medullary shell 0.04; pores of the inner cortical shell 0.006, bars 0.003.

_Habitat._--South Pacific, Station 295, depth 1500 fathoms.

2. _Staurotholonium bicruciatum_, n. sp.

Outer cortical shell rough, twice as large as the inner, of the same form and structure, at an equal distance from it. In both shells the principal cupolas are larger than the lateral. Pores irregular, roundish, in the outer shell four times, in the inner twice as large as the bars; eight to ten in the basal semicircle of one cupola. (Similar to _Staurotholus cruciatus_, but differs mainly in the double cortical shell.) Medullary shell lentelliptical.

_Dimensions._--Major (longitudinal) axis of the outer cortical shell 0.016, of the inner 0.08; minor (transverse) axis of the former 0.14, of the latter 0.07; medullary shell 0.03 to 0.04.

_Habitat._--Pacific, central area, Station 267, depth 2700 fathoms.

3. _Staurotholonium alternatum_, n. sp.

Outer cortical shell smooth, very different from the inner; in the outer the two principal cupolas are larger than the two lateral; in the inner inversely smaller. Pores subregular, circular, in the outer shell four times, in the inner shell twice as broad as the bars; ten to twelve pores in the basal semicircle of one cupola.

{676}_Dimensions._--Major axis (length) of the outer cortical shell 0.15, minor (breadth) 0.13; pores 0.012, bars 0.003; major axis (length) of the inner cortical shell 0.11, minor axis (breadth) 0.09; pores 0.006, bars 0.003.

_Habitat._--Indian Ocean, surface, Madagascar, Rabbe.

4. _Staurotholonium lenticulare_, n. sp.

Outer cortical shell smooth, lenticular, with circular circumference, with very delicate irregular network and small roundish pores. Its distance from the inner much greater in the four diagonal points than in the four polar points. Inner cortical shell regular, cross-like, with four cupolas of similar size and form. Pores subregular, circular, twice as broad as the bars; eight to ten in the basal semicircle of one cupola. Medullary shell lenticular.

_Dimensions._--Diameter of the outer cortical shell 0.16, of the inner 0.12; pores of the inner 0.008, bars 0.004.

_Habitat._--North Atlantic, Station 353, surface.

Subgenus 2. _Staurotholoma_, Haeckel.

_Definition._--Surface of the outer cortical shell with radial spines or thorns.

5. _Staurotholonium octodoratium_, n. sp.

Outer cortical shell of the same form and structure as the inner, but twice as large, both principal domes somewhat larger than both lateral. Pores subregular, circular, of the same breadth as the bars; five to six in the semicircle of one inner, eight to ten of one outer cupola. Eight thin and long bristle-like spines, opposite in pairs in two crossed diagonal planes.

_Dimensions._--Major axis of the outer cortical shell 0.15, minor 0.12; major axis of the inner cortical shell 0.07, minor 0.06; pores and bars 0.005.

_Habitat._--North Pacific, Station 231, surface.

6. _Staurotholonium octodoronium_, n. sp. (Pl. 10, fig. 11).

Outer cortical shell of the same form as the inner, both little distant; network of the outer irregular and delicate. Pores of the inner regular, circular, twice as broad as the bars; seven to nine in the basal semicircle of one cupola. Both lateral domes larger than the principal. Eight long and thin, cylindrical radial spines opposite in pairs in two crossed diagonal planes.

_Dimensions._--Major axis of the outer cortical shell 0.2, minor 0.18; major axis of the inner cortical shell 0.16, minor 0.14; pores 0.008, bars 0.004.

_Habitat._--South Pacific, Station 288, surface.

{677}Subfamily 3. CUBOTHOLIDA, Haeckel.

_Definition._--#Tholonida# with triaxial growth; cupolas lying in pairs on the six sides of a cubical central chamber, opposite at the poles of three axes perpendicular one to another. (Shell commonly seven-chambered, with six domes surrounding the central chamber.)

Genus 297. _Tholocubus_,[337] n. gen.

_Definition._--#Tholonida# with simple cortical shell (without external veil), composed of six hemispherical cupolas, opposite in pairs on the poles of three axes perpendicular one to another, covering six sides of the simple cuboidal central chamber (without medullary shell).

The genus _Tholocubus_ is the most simple form of the Cubotholida, or of the Tholonida with domes situated in three axes perpendicular one to another; six hemispherical cupolas lying on the six sides of a cuboidal central chamber; this latter contains no medullary shell. _Tholocubus_ may be derived phylogenetically either from _Tholostaurus_ by apposition of two opposite domes on the flat sides of the cross-shell, or from _Cubotholus_ by loss of the medullary shell.

Subgenus 1. _Tholocubulus_, Haeckel.

_Definition._--Surface of the shell smooth or rough, without radial spines.

1. _Tholocubus tessellatus_, n. sp. (Pl. 10, fig. 12).

Surface of the shell smooth, without radial spines. Pores regular, circular, hexagonally framed, twice as broad as the bars; eight to twelve pores on the semicircle of one cupola. Principal cupolas smaller than the lateral, larger than the sagittal cupolas.

_Dimensions._--Diameter of the shell 0.15; pores 0.01, bars 0.005.

_Habitat._--South Atlantic, Station 332, depth 2200 fathoms.

2. _Tholocubus tesserarius_, n. sp.

Surface of the shell rough, without radial spines. Pores irregular, roundish, once to three times as broad as the bars; twelve to sixteen pores in the semicircle of one cupola. All six cupolas nearly of the same size.

_Dimensions._--Diameter of the shell 0.16; pores 0.004 to 0.008, bars 0.003.

_Habitat._--North Atlantic, Station 353, depth 2965 fathoms.

{678}Subgenus 2. _Tholocubitus_, Haeckel.

_Definition._--Surface of the shell with radial spines or thorns.

3. _Tholocubus tesseralis_, n. sp. (Pl. 10, fig. 16).

Surface of the shell with numerous (eight to sixteen) thin and long, bristle-shaped radial spines (the greater part broken off in the figured specimen); pores subregular, circular, three to four times as broad as the bars; ten to twelve in the semicircle of one cupola.

_Dimensions._--Major axis of the shell 0.16, minor axis 0.14; pores 0.01, bars 0.0027.

_Habitat._--Pacific, central area, Station 271, depth 2425 fathoms.

Genus 298. _Tholonium_,[338] n. gen.

_Definition._--#Tholonida# with double cortical shell (with external veil), composed of six hemispherical cupolas, opposite in pairs on the poles of three axes perpendicular one to another, covering six sides of the simple cuboidal central chamber (without medullary shell).

The genus _Tholonium_ (Pl. 10, fig. 17) differs from the preceding _Tholocubus_ only in the duplication of the cortical shell. The outer (secondary) shell forms either a simple (spheroidal or ellipsoidal) thin veil around the inner (primary) shell, or both shells are of the same form, with six corresponding cupolas. Possibly _Tholonium_ is the offspring of _Cubotholonium_, having originated by loss of the medullary shell.

Subgenus 1. _Tholonetta_, Haeckel.

_Definition._--Surface of the shell smooth or rough, without radial spines or thorns.

1. _Tholonium bicubicum_, n. sp.

Outer shell with six hemispherical dome-shaped protuberances, corresponding in those of the inner shell. Both shells connected by numerous radial beams. Surface of the outer shell smooth; its network nearly of the same shape as that of the inner, with subregular, circular pores, twice as broad as the bars; ten to fifteen pores on the semicircle of one cupola.

_Dimensions._--Diameter of the outer shell 0.14, of the inner 0.12; pores of the latter 0.006, bars 0.003.

_Habitat._--South Atlantic, Station 325, depth 2650 fathoms.

{679}2. _Tholonium ellipticum_, n. sp.

Outer shell ellipsoidal, without dome-shaped protuberances, with smooth surface, without radial spines; network delicate, with subregular, circular pores. Inner shell with regular, circular, hexagonally framed pores, twice as broad as the bars; ten to twelve pores on the semicircle of one cupola.

_Dimensions._--Major axis of the outer shell 0.16, minor axis 0.14; major axis of the inner shell 0.14, minor axis 0.12; pores 0.008, bars 0.004.

_Habitat._--Pacific central area, Station 267, depth 2700 fathoms.

3. _Tholonium sphæricum_, n. sp.

Outer shell spherical, without dome-shaped protuberances, with smooth surface, without radial spines; network very delicate, with very small subregular, circular pores. Inner shell with regular, circular pores of the same breadth as the bars; fourteen to sixteen on the basal semicircle of one cupola.

_Dimensions._--Diameter of the spherical outer shell 0.15, inner shell 0.12; pores and bars of the inner shell 0.005.

_Habitat._--Indian Ocean, Zanzibar, Pullen, depth 2200 fathoms.

Subgenus 2. _Tholonilla_, Haeckel.

_Definition._--Surface of the outer shell covered with radial spines or thorns.

4. _Tholonium hexonium_, n. sp. (Pl. 10, fig. 17).

Outer shell ellipsoidal, without dome-shaped protuberances, covered with numerous (thirty to fifty) thin and long, bristle-shaped radial spines; network very delicate, irregular, with roundish pores. Inner shell with six marked hemispherical domes of somewhat different sizes; pores regular, circular, with prominent hexagonal frames, twice as broad as the bars; ten to twelve on the basal semicircle of one cupola.

_Dimensions._--Major axis of the outer shell 0.15, minor axis 0.14; major axis of the inner shell 0.13, minor axis 0.12; pores 0.008, bars 0.004.

_Habitat._--Pacific, central area, Station 271, depth 2425 fathoms.

5. _Tholonium sphæronium_, n. sp.

Outer shell very thin walled, spherical, without dome-shaped protuberances, with smooth surface, excepting eight large radial spines, rising from the eight corners of the inner cubical central chamber. Pores of the outer shell very small, subcircular. Inner shell very thick walled, with six marked hemispherical domes; pores subregular, circular, with prominent hexagonal frames, twice as broad as the bars; six to eight on the semicircle of one cupola.

{680}_Dimensions._--Diameter of the spherical outer shell 0.15, inner shell 0.11; pores of the inner 0.01, bars 0.005.

_Habitat._--Pacific, central area, Station 274, depth 2750 fathoms.

Genus 299. _Cubotholus_,[339] n. gen.

_Definition._--#Tholonida# with simple cortical shell (without external veil), composed of six hemispherical cupolas, opposite in pairs on the poles of three axes perpendicular one to another, covering six sides of the cuboidal _Larnacilla_-shaped central chamber (with medullary shell).

The genus _Cubotholus_ differs from _Tholocubus_ in the possession of a medullary shell in the central chamber, and may be derived from this genus by its production; but it may also be derived from _Staurotholus_ by apposition of two opposite domes on the flat sides of the cross-shell. Sometimes all six domes are of the same size and form, but commonly different in pairs.

Subgenus 1. _Cubotholissa_, Haeckel.

_Definition._--Surface of the shell smooth or rough, without radial spines.

1. _Cubotholus regularis_, n. sp. (Pl. 10, fig. 14).

On the six sides of the cubical central chamber six hemispherical cupolas of the same size and form. Surface smooth. Pores regular, circular, twice as broad as the bars; eight in the basal semicircle of each cupola. Medullary shell apparently spherical (?), connected with the eight corners of the central chamber by eight radial beams, regularly disposed. (This species is remarkable for the perfect symmetry of the shell, the six sides of which appear to be quite similar. It differs from the similar _Tholocubus regularis_ in the possession of a medullary shell.)

_Dimensions._--Diameter of the cortical shell (equal in all three dimensive axes) 0.15; pores 0.01, bars 0.005; diameter of the medullary shell 0.04.

_Habitat._--Pacific, central area, Station 273, depth 2350 fathoms.

2. _Cubotholus quadraticus_, n. sp.

Surface of the shell smooth. Both principal cupolas (on the poles of the longitudinal axis) larger than the four other cupolas, which have the same size and lie cross-wise in the equatorial plane. (Therefore two of the three fundamental axes equal, the third larger.) Pores subregular, circular, three times as broad as the bars; ten to twelve in the semicircle of each cupola-basis.

_Dimensions._--Major axis 0.16, minor axis 0.12; pores 0.012, bars 0.004.

_Habitat._--Indian Ocean, Zanzibar, Pullen; depth 2200 fathoms.

{681}3. _Cubotholus rhombicus_, n. sp.

Surface of the shell smooth. Both principal cupolas (on the poles of the longitudinal axis) larger than the two lateral (on the poles of the transverse axis), and these larger than the two sagittal cupolas (on the poles of the sagittal axis). Therefore all three fundamental axes unequal. Pores irregular, roundish, twice to four times as broad as the bars; eight to twelve in the basal semicircle of each cupola. Medullary shell lentelliptical.

_Dimensions._--Major (principal) axis 0.16, middle (lateral) axis 0.14, minor (sagittal) axis 0.12; pores 0.006 to 0.012, bars 0.003.

_Habitat._--South Atlantic, Station 335, depth 1425 fathoms.

Subgenus 2. _Cubotholura_, Haeckel.

_Definition._--Surface of the shell with radial spines or thorns.

4. _Cubotholus octoceras_, n. sp.

All six cupolas in pairs of different sizes. Both principal domes larger than the lateral domes, and these larger than the sagittal domes. Pores subregular, circular, twice as broad as the bars; eight to ten in the semicircle of each cupola. From the surface arise (at the intersecting points of every three cupolas) eight strong conical radial spines, about as long as the shell-axis; they lie in two diagonal planes, and are the external free prolongations of eight inner beams (homologous with the eight wing-spines of _Tetrapyle octacantha_), which connect the lentelliptical medullary shell with the eight corners of the cuboidal central chamber.

_Dimensions._--Major axis of the cortical shell 0.15, middle 0.13, minor 0.11; pores 0.008, bars 0.004; diameters of the medullary shell corresponding to 0.05, 0.04, 0.03.

_Habitat._--Western Tropical Pacific, Station 224, depth 1850 fathoms.

Genus 300. _Cubotholonium_,[340] n. gen.

_Definition._--#Tholonida# with double (or sometimes triple) cortical shell (with external veil), composed of six hemispherical cupolas, opposite in pairs on the poles of three axes perpendicular one to another, covering six sides of the cuboidal _Larnacilla_-shaped central chamber (with medullary shell).

The genus _Cubotholonium_ differs from the nearly allied _Cubotholus_ (its probable ancestral form) only in duplication of the cortical shell. I have observed only two species of this rare form, both rather different. In the first species the outer cortical shell forms a simple spherical thin veil around the inner, the six cupolas of which are nearly of the same form and size. In the second species each of the domes of the {682}inner cortical shell is protected by an outer larger cupola, and besides this the whole shell is enveloped by a thin ellipsoidal veil (Pl. 10, fig. 15). Therefore this species may be the representative of a peculiar genus, the most highly developed of all Tholonida--_Tholothauma_.

1. _Cubotholonium sphæroides_, n. sp.

Outer cortical shell (or veil) spherical, with smooth surface; network very delicate, with very thin bars and very small irregular, roundish pores. Inner cortical shell simple, composed of six hemispherical cupolas, surrounding the six sides of the cubical central chamber, which encloses a spherical medullary shell (one-third as large as itself). Network of the inner cortical shell regular, with circular pores of the same breadth as the bars, twelve to fourteen in the basal semicircle of one cupola.

_Dimensions._--Diameter of the spherical outer shell 0.2, of the inner cortical shell 0.15; pores and bars of the latter 0.006.

_Habitat._--Indian Ocean, Zanzibar, Pullen, depth 2200 fathoms.

2. _Cubotholonium ellipsoides_, n. sp. (Pl. 10, fig. 15).

_Tholothauma ellipsoides_, Haeckel, 1883, MS.

Outer cortical shell (or veil) ellipsoidal, with very thin irregular network and thorny surface. Inner cortical shell double, with six double, flatly vaulted cupolas, surrounding the six sides of the _Larnacilla_-shaped central chamber; the double domes of each shell are in opposite pairs somewhat larger than the alternating pairs. Pores subregular, circular, about the same breadth as the bars; eight to twelve in the basal semicircle of one cupola. Central chamber with ellipsoidal medullary shell. Radial spines short, very numerous.

_Dimensions._--Major axis of the outer cortical shell 0.28, minor 0.24; major axis of the inner cortical shell 0.16, minor axis 0.14; pores and bars 0.006; medullary shell 0.03.

_Habitat._--Pacific, central area, Station 271, depth 2425 fathoms.

Family XXVIII. #ZONARIDA#, n. fam. (Pl. 50, figs. 9-12).

_Definition._--#Larcoidea# with regular, completely latticed cortical shell, distinguished by two to four or more annular constrictions, which lie (all or partly) in the dimensive planes (sagittal, transverse, or lateral), and by which four to eight or more vaulted cupolas or dome-like chambers become separated. In the centre of this chambered cortical shell lies constantly a trizonal or _Larnacilla_-shaped medullary shell.

The family #Zonarida# comprises a small number of peculiar #Larcoidea#, resembling the Tholonida in the composition of the polythalamous cortical shell by a number of cupolas or dome-shaped protuberances. But the disposition and origin of these latter {683}are quite different. Whilst in the Tholonida the axes of the domes are dimensive axes, and these are separated by annular constrictions lying in diagonal planes, in the Zonarida we find the contrary; the axes of the domes are here diagonal axes, and these are separated by annular constrictions lying in dimensive planes. However, this definition agrees absolutely only in the four-chambered _Zonarium_ and in the eight-chambered _Zonidium_, whilst in the six chambered _Zoniscus_ only four domes are disposed according to this law, two others, however, in the same manner as in the Tholonida. Therefore this genus is intermediate between both families.

_The Cortical Shell_ of the Zonarida is in all cases completely latticed and of regular lentelliptical fundamental form, as in the nearly allied Larnacida and Tholonida. The three dimensive axes are constantly of different sizes, each with two equal poles; commonly (as in the human body) the principal or longitudinal axis is the longest, the sagittal (or dorso-ventral) axis the shortest; the transverse (or lateral) axis being intermediate between them. Of the three dimensive planes the lateral plane is the largest (determined by the principal and transverse axes); the smallest is the equatorial plane (crossed by the transverse and sagittal axes); the sagittal plane (determined by the sagittal and principal axes) being intermediate between them.

The annular constrictions of the cortical shell which produce the dome-shaped protuberances are different in number in the three known genera--two, three, or four. To each constriction often (but not always) corresponds an internal latticed septum, which connects the cortical with the medullary shell. The number of the cupolas is always double the number of the annular constrictions by which they are separated, therefore four, six, or eight.

In all known Zonarida the sagittal septum is quite constant, but derived from the original axial rod, which lies in the principal axis. By ramification of this axial beam and reticular connection with the sagittal girdle arises the sagittal septum, which we found first in _Octopyle_, halving the four gates of _Tetrapyle_. Whilst this sagittal septum (between right and left halves of the body) is common to all three known genera of this family, the number and shape of the other annular constrictions are different. In _Zonarium_ (Pl. 50, fig. 9) we find only one transverse constriction (in the equatorial plane), in _Zoniscus_ (figs. 10, 11) two parallel transverse constrictions (parallel to the equatorial plane, on both sides of it). _Zonidium_ (fig. 12) is a combination of both foregoing genera; it has three parallel transverse constrictions (one in the equatorial plane, and one on each side of it).

_The Latticed Domes_ (cupolas or chambers) of the cortical shell exhibit correspondingly a different number and disposition in the three known genera. In _Zonarium_ are found only four crossed chambers, separated by the sagittal and transverse septa; the axes of the four crossed domes are diagonal axes, whilst in the similar _Staurotholus_ they are dimensive axes (principal and transverse axes). In _Zonidium_ we find eight domes, each cupola of _Zonarium_ being halved by a diagonal septum. _Zoniscus_ is intermediate {684}between the two foregoing genera, and has six cupolas, three on each side of the sagittal septum.

The inner communication of the cupolas or chambers is more or less free, the lattice-work of the separating septa between them commonly remaining more or less imperfect, or represented only by some isolated beams or meshes. The outer network of the cupolas is commonly irregular (as in the majority of #Larcoidea#), but sometimes distinguished by a small number of regularly disposed larger apertures (similar to the "gates" of the Pylonida). From the surface radial spines often arise in characteristic number and symmetrical disposition, commonly as prolongations of the septal axes or of the constricted edges.

The _Medullary Shell_ in all Zonarida is a true trizonal or _Larnacilla_-shaped lattice-shell (compare above, p. 600); its perimeter (or the first lateral girdle) is sometimes more elliptical, at other times more hexagonal; the hexagon is amphithect; both its lateral sides are often concave and commonly longer than the four other sides.

The _Central Capsule_ in all Zonarida is in a strict geometrical sense a true lentellipsis (compare above, p. 599); its principal axis is commonly one and a third to one and a half times as great as the transverse axis, and twice to three times as great as the sagittal axis. The lentelliptical central capsule encloses the trizonal medullary shell, whilst it is externally enveloped by the chambered cortical shell.

_Synopsis of the Genera of Zonarida._

Two annular constrictions and four cupolas, 301. _Zonarium_.

Three annular constrictions and six cupolas, 302. _Zoniscus_.

Four annular constrictions and eight cupolas, 303. _Zonidium_.

Genus 301. _Zonarium_,[341] n. gen.

_Definition._--#Zonarida# with four dome-shaped chambers of the cortical shell, separated by two annular constrictions (one sagittal and one transverse).

The genus _Zonarium_ is the most simple form of the Zonarida, and differs from the nearly allied _Larnacalpis_ by two ring-like constrictions, which are crossed at right angles, one in the sagittal (or median) plane, and one in the transverse (or equatorial) plane. By these two annular constrictions four egg-shaped or kidney-shaped chambers become imperfectly separated, which correspond to the quadrants of the lateral plane. The first cause of the marked constrictions may be the formation of the latticed sagittal septum, which is found first in _Octopyle_, as halving the four gates of {685}_Tetrapyle_. Between this septum and the constricted narrow transverse girdle the cortical shell grows out in the form of four vaulted cupolas; every two opposite domes are congruent, two neighbouring are symmetrically equal.

1. _Zonarium quadrigatum_, n. sp.

Cortical shell quadrangular, one and a half times as long as broad, with four rounded corners. Surface thorny, with numerous short radial spines. Sagittal constriction twice as long as the hexagonal medullary shell. Four cupolas kidney-shaped.

_Dimensions._--Length of the cortical shell 0.16, breadth 0.11; length of the medullary shell 0.06, breadth 0.04.

_Habitat._--Pacific, central area, Station 272, depth 2600 fathoms.

2. _Zonarium quadrispinum_, n. sp.

Cortical shell quadrangular, nearly rectangular, about twice as long as broad, with four corners, from which arise four strong, three-sided pyramidal, radial spines (crossed in two diagonals of the lateral plane). Surface thorny, only smooth in the transverse constriction. Sagittal constriction three times as long as the lentelliptical medullary shell. Four cupolas elliptical or nearly quadrangular.

_Dimensions._--Length of the cortical shell 0.22, breadth 0.1; length of the medullary shell 0.07, breadth 0.05.

_Habitat._--Pacific, central area, Station 274, depth 2750 fathoms.

3. _Zonarium octangulum_, n. sp. (Pl. 50, fig. 9).

Cortical shell octangular, one and a fourth times as long as broad, with eight strong conical spines on the eight corners; these are separated by four deep constrictions on the poles of the principal and transverse axes, and by four truncated planes on the poles of the crossed diagonal axes between the former. Sagittal constriction twice as long as the hexagonal medullary shell. Four cupolas kidney-shaped or nearly pentagonal.

_Dimensions._--Length of the cortical shell 0.15, breadth 0.12; length of the medullary shell 0.07, breadth 0.04.

_Habitat._--Pacific, central area, Station 271, depth 2425 fathoms.

4. _Zonarium tetratholium_, n. sp.

Cortical shell quadrangular, with four rounded corners. Surface thorny, with sixteen stronger radial spines; eight of these lie in the lateral plane, in the same symmetrical disposition as in the foregoing species; eight others lie on both sides of the lateral plane, opposite in pairs in two crossed diagonal planes, in the same symmetrical disposition as in _Tetrapyle octacantha_. Sagittal {686}constriction three times as long as the lentelliptical medullary shell. Four cupolas obliquely elliptical.

_Dimensions._--Length of the cortical shell 0.16, breadth 0.12; length of the medullary shell 0.04, breadth 0.03.

_Habitat._--North Pacific, Station 241, depth 2300 fathoms.

Genus 302. _Zoniscus_,[342] n. gen.

_Definition._--#Zonarida# with six dome-shaped chambers of the cortical shell, separated by three annular constrictions (one sagittal and two transverse, parallel to the equatorial plane).

The genus _Zoniscus_ differs from _Zonarium_ as well as from _Zonidium_ by the development of the transverse girdle, which is not constricted, but on the contrary prominently vaulted in the equatorial plane. Therefore both wings of the transverse girdle form here two opposite lateral or "equatorial cupolas," as in _Amphitholus_. These are separated from four other domes (the "corner cupolas") by two transverse annular constrictions, which correspond to the free edges of the original transverse girdle. The corner domes of each pair are separated from each other by the sagittal septum.

1. _Zoniscus rectangulus_, n. sp.

Cortical shell nearly rectangular, with rounded corners, nearly one and a half times as long as broad. Surface thorny, without larger radial spines. Sagittal constriction scarcely half as long as the hexagonal medullary shell. Both equatorial cupolas (or wings of the transverse girdle) scarcely half as large as the four corner cupolas.

_Dimensions._--Length of the cortical shell 0.15, breadth 0.11; length of the medullary shell 0.07, breadth 0.045.

_Habitat._--North Pacific, Station 253, depth 3125 fathoms.

2. _Zoniscus hexathalamus_, n. sp.

Cortical shell nearly elliptical, one and a third times as long as broad. Surface nearly smooth, without radial spines. Sagittal constriction three times as long as the lentelliptical medullary shell. Both equatorial cupolas about as large as the four corner cupolas.

_Dimensions._--Length of the cortical shell 0.12, breadth 0.09; length of the medullary shell 0.035, breadth 0.025.

_Habitat._--North Pacific, Station 244, depth 2900 fathoms.

{687}3. _Zoniscus tetracanthus_, n. sp. (Pl. 50, fig. 10).

Cortical shell nearly quadrangular, one and a half times as long as broad, with four prominent corners, from which arise in the lateral plane four strong, three-sided pyramidal radial spines (opposite in pairs in two crossed diagonal axes). Surface thorny, with exception of the concave lateral sides. Sagittal constriction two and a half times as long as the hexagonal medullary shell. Both equatorial cupolas scarcely half as large as the four corner cupolas.

_Dimensions._--Length of the cortical shell 0.18, breadth 0.12; length of the medullary shell 0.07, breadth 0.04.

_Habitat._--Pacific, central area, Station 271, depth 2425 fathoms.

4. _Zoniscus octacanthus_, n. sp.

Cortical shell nearly four-sided, prismatic, one and a third times as long as broad, with spiny surface. Eight longer thin radial spines opposite in pairs in two crossed diagonal planes (as in _Tetrapyle octacantha_). Sagittal constriction two and a half times as long as the hexagonal medullary shell. Both equatorial cupolas somewhat larger than the four corner cupolas.

_Dimensions._--Length of the cortical shell 0.16, breadth 0.12; length of the medullary shell 0.055, breadth 0.04.

_Habitat._--Pacific, central area, Station 263, depth 2650 fathoms.

5. _Zoniscus hexatholius_, n. sp. (Pl. 50, fig. 11).

Cortical shell nearly rectangular, with four rounded corners and deep sagittal constriction, one and a fourth times as long as broad with thorny surface. Twelve longer edged radial spines; eight opposite in pairs in two crossed diagonal planes (as in the foregoing species), four others in the lateral plane, opposite in pairs on both sides of the sagittal constriction, which is scarcely twice as long as the hexagonal, in the equatorial plane constricted medullary shell. Both equatorial cupolas nearly of the same size as the four corner cupolas.

_Dimensions._--Length of the cortical shell 0.16, breadth 0.13; length of the medullary shell 0.07, breadth 0.04.

_Habitat._--South Pacific, Station 300, depth 1375 fathoms.

Genus 303. _Zonidium_,[343] n. gen.

_Definition._--#Zonarida# with eight dome-shaped chambers of the cortical shell, separated by four annular constrictions (one sagittal, one transverse, and two others parallel to the latter).

The genus _Zonidium_ must be regarded as a _Zonarium_, in which the four cupolas (in the quadrants of the lateral plane) are halved by diagonal beams or latticed septa, {688}lying opposite in pairs in two crossed diagonal planes. Therefore the number of the septa (four) and the domes (eight) is here doubled. Among the eight cupolas we distinguish four median (on both sides of the sagittal plane) and four lateral (on both sides of the equatorial plane); both groups are of different size and form. Probably _Zonidium_ is derived from _Zonarium_ by lattice-connection between the eight diagonal wing-spines, which in both species of this genus are present, the same as in _Tetrapyle octacantha_.

1. _Zonidium octostylium_, n. sp.

Cortical shell nearly quadrangular, with rounded corners and thorny surface. Eight long and thin radial spines on both sides of the lateral plane opposite in pairs and lying in two crossed diagonal planes (as in _Tetrapyle octacantha_). Sagittal constriction three times as long as the lentelliptical medullary shell. Four medial cupolas (on both sides of the sagittal plane) somewhat larger than the four lateral cupolas (on both sides of the equatorial plane).

_Dimensions._--Length of the cortical shell 0.15, breadth 0.12; length of the medullary shell 0.05, breadth 0.03.

_Habitat._--Indian Ocean, Madagascar, Rabbe, surface.

2. _Zonidium octotholium_, n. sp. (Pl. 50, fig. 12).

Cortical shell nearly octangular, with spiny surface; twenty long and stout radial spines between numerous smaller spines; eight wing-spines opposite in pairs in two crossed diagonal planes (as in the preceding species); twelve other strong spines in the lateral plane (four longer opposite in pairs on the poles of the principal and transverse axes, eight others smaller, alternating between these and the diagonal spines). Sagittal constriction twice as long as the hexagonal medullary shell. Four median cupolas somewhat smaller than the four lateral cupolas.

_Dimensions._--Length of the cortical shell 0.18, breadth 0.15; length of the medullary shell 0.07, breadth 0.05.

_Habitat._--Indian Ocean, Zanzibar, Pullen, depth 2200 fathoms.

Family XXIX. #LITHELIDA#, Haeckel (Pl. 49, figs. 1-7).

_Lithelida_, Haeckel, 1862, Monogr. d. Radiol., p. 515.

_Definition._--#Larcoidea# with symmetrical spiral shell, divided by the spiral plane into two symmetrical halves; all windings of the spiral lie in this plane. Primordial chamber either simple or _Larnacilla_-shaped.

The family #Lithelida# comprises all those #Larcoidea# in which the growth of the latticed shell is spirally winding in one plane, _Nautilus_-like. They agree in the spiral growth with the following family. But in the Streblonida the spiral is screw-shaped, {689}ascending (like _Helix_). Therefore in these latter the geometrical fundamental form of the shell is asymmetrical or "dysdipleural," whereas in the Lithelida bilateral-symmetrical or "eudipleural." The lentelliptical or nearly spherical shell may be divided by a median section into two symmetrical halves; the right half is the mirror image of the left half.

When in 1862 I founded the family Lithelida in my Monograph (p. 515), I knew only one genus, _Lithelius_, with two species. The rich material of the Challenger collection contains a great number of similar spirally constructed #Larcoidea#, so that at the present time we may distinguish at least six genera. These belong to two different subfamilies, which may possibly be afterwards better separated as families. The first subfamily, Spiremida, possess a simple, spherical or subspherical, medullary shell; the second subfamily, Larcospirida, possess a trizonal or _Larnacilla_-shaped medullary shell. No doubt these latter must be derived from Pylonida, as we observe all stages of development starting from a simple _Trizonium_; but perhaps also the Spiremida have the same origin, their simple, spherical or subspherical, medullary shell being derived from a trizonal or _Larnacilla_-shaped medullary shell by reduction.

The general appearance in both subfamilies of the Lithelida is quite the same, and it requires a careful study of the medullary shell to distinguish certainly the Spiremida from the Larcospirida. This distinction is often not easy, particularly in the larger forms; the shell is often very opaque and difficult to understand. Only in one position, if the spiral axis be parallel to the axis of the eye of the observer, and the spiral plane be therefore fully seen in the optical plane of the microscope, the spiral line (or the axial section of the latticed spiral lamella) is distinctly observed; in all other positions the figure of the spiral is more or less indistinct, and the whole microscopical image often quite intricate and confused. The sufficient study of this family requires therefore the contemplation of the shell from different sides, and is the more difficult, as the variability of the Lithelida--as of the Pylonida--is extraordinarily great.

The description which I gave of _Lithelius_ (1862) in my Monograph is in some points erroneous, and was afterwards (1879) corrected by R. Hertwig, who explained particularly the near relation of it to _Tetrapyle_. Indeed the intermediate forms between the Lithelida and the Pylonida are so numerous and so evident in all stages of development, that the derivation of the former (at least of the Larcospirida) from the latter is quite clear. The analogy between the structure of the Lithelida and the calcareous (foraminiferous) Alveolinida is not so complete as I supposed it to be in my Monograph (1862); particularly the formation of the small chambers between the turnings of the spiral lamella is much more complete in the Alveolinida than in the Lithelida.

The cortical shell of all Lithelida has the same geometrical fundamental form as _Nautilus_ or as the nautiloid Polythalamia (_Polystomella_, _Nummulites_, &c.); therefore the shell is dipleural, being divided by the median plane into two symmetrical lateral halves. {690}Since the spiral line lies in the median plane, we will call it the spiral plane; it separates the right half from the left. The axis of the body, around which the spiral turns (without touching it), is the spiral axis. The latticed part of the cortical shell, which turns around them, is the spiral lamella. Only in one genus of our family, viz., _Tholospira_, are the spiral axis, the lateral axis, the spiral plane, and the sagittal plane quite as in _Nautilus_. In all other genera this disposition is different or is uncertain. This depends on the different part of the cortical shell, from which the spiral growth begins. In this respect we can distinguish four different modes.

In the Larcospirida (or the Lithelida with _Larnacilla_-shaped medullary shell) the spiral growth exhibits four quite different forms. It begins here with _Larcospira_, in which already the first cortical girdle of the Diplozonaria determines the spiral growth; one wing of this girdle, the transverse girdle of _Amphipyle_, grows more swiftly than the other, overgrows it, and thus turns around the principal axis. In _Pylospira_ the first or transverse girdle is already perfectly formed (as in _Amphipyle_), and the spiral growth is introduced by the second or lateral girdle of _Tetrapyle_; one wing of it (the right or the left) grows more swiftly than the other, overgrows it, and thus turns around the sagittal axis. In _Tholospira_ also the second girdle is complete, and the spiral growth begins from the third or sagittal girdle. One of its wings grows more swiftly than the other, overgrows it, and thus turns around the transverse axis. Consequently we see that each of the three dimensive planes of the lentelliptical Larcoid-body may be the spiral plane: in _Larcospira_ the transverse plane, in _Pylospira_ the lateral plane, in _Tholospira_ the sagittal plane. Correspondingly the spiral axis in the first genus is the principal, in the second the sagittal, in the third the transverse axis of the central _Larnacilla_-shell. Therefore in these three genera the spiral plane is the plane of the latticed girdle, which determines the spiral growth, one of both its wings overgrowing the other.

In each of the three above mentioned genera the spiral may be simple or double; it remains simple if only one of both wings of the turning girdle overgrow the other, and this latter remain a simple half girdle (or tube-like wing). Whereas the spiral becomes double if the second wing of the girdle afterwards follow the example of the first wing and now turn around it in the same direction. As this happens in all three genera, we can subdivide them into six subgenera.

A quite peculiar form of spiral growth is produced in _Spironium_, in which the direction of growth in both lateral wings of the transverse girdle is inverse from the beginning. The left wing grows against the posterior, the right wing against the anterior pole of the principal axis, turning around it in crossed, eight-like spirals. The whole shell afterwards assumes a lentelliptical form.