Chapter XXXVIII: Section IV: STICHOCYRTIDA, Haeckel, 1862, Monogr. d. Radiol., p. 280, 312 (13)

a = Depth in Fathoms.
b = Challenger Station.
c = Length of the Shell.
d = Breadth of the Shell.
e = Number of Feet.
f = Number of Pedal Pores.
g = Number of Teeth.
h = Number of Dental Pores.
+------------------------------+------+-----+-----+-----+---+---+---+---+
| | a | b | c | d | e | f | g | h |
+------------------------------+------+-----+-----+-----+---+---+---+---+
| 1. _Tuscarora bisternaria_, | 3000 | 264 | 2.0 | 1.5 | 3 | 8 | 3 | 8 |
| 2. " _murrayi_, | 2000 | 295 | 2.5 | 1.5 | 3 | 3 | 3 | 3 |
| 3. " _wyvillei_, | 2250 | 291 | 1.5 | 1.4 | 3 | 4 | 3 | 4 |
| 4. " _tetrahedra_, | 2450 | 348 | 2.5 | 2.0 | 3 | 4 | 3 | 3 |
| 5. " _tubulosa_, | 3000 | 249 | 1.4 | 1.2 | 3 | 4 | 2 | 3 |
| 6. " _porcellana_, | 2650 | 325 | 1.5 | 1.3 | 3 | 4 | 2 | 4 |
| 7. " _belknapii_, | 2025 | 293 | 2.5 | 1.5 | 3 | 3 | 4 | 2 |
| 8. _Tuscarusa medusa_, | 3125 | 253 | 1.2 | 1.0 | 4 | 4 | 2 | 2 |
| 9. _Tuscaridium cygneum_, | 3050 | 250 | 3.2 | 1.6 | 1 | 2 | 4 | 4 |
|10. " _lithornithium_, | 3000 | 264 | 3.6 | 1.8 | 1 | 4 | 4 | 6 |
+------------------------------+------+-----+-----+-----+---+---+---+---+

The base of the apophyses in all Tuscarorida is inflated, conical, and pierced by a small number of large ovate pores, the typical "basal pores," which occur also in the closely allied Circoporida. The number of these basal pores varies from two to eight, and is usually three or four; it never becomes in this family so great as in the Circoporida, where each circle of pores is often composed of sixteen to twenty-four or more basal pores. The number seems to be rather constant in each single species, as may be seen in the preceding Table. The pedal pores (on the base of the aboral feet) are usually larger than the dental pores (on the base of the circoral teeth). Their form is {1705}usually irregularly ovate or triangular; their outer aperture is armed with spines or bristles, which are commonly larger than in the other parts of the apophyses.

The mouth of the shell varies in form, according to the number and arrangement of the teeth on its corners. It is therefore a narrow transverse fissure, with two broad opposite lips and two corners, in the bidental forms (Pl. 100, figs. 5, 7), triangular in the tridental species (figs. 1-4), quadrangular or square in the quadridental species, _Tuscarora belknapii_ (Narr. Chall. Exp., _loc. cit._, pl. A, fig. 15). The singular genus _Tuscaridium_ (fig. 8) exhibits four teeth, which are nearly horizontally divergent in two pairs, a dorsal and a ventral pair (corresponding in position to the four feet of _Tuscarusa_, fig. 7); the mouth is here prolonged into a cylindrical, spinulate proboscis, which is curved towards the ventral face of the shell (fig. 8).

The _central capsule_ of the Tuscarorida is kidney-shaped or spheroidal, scarcely half as large as the dark olive-green phæodium, which surrounds its anterior (oral) face. Usually the capsule and the phæodium together fill up the aboral half of the shell-cavity, and are separated from its walls by the calymma. The latter is pierced by numerous branched and reticulately anastomosing pseudopodia, which arise from the matrix enveloping the capsule, and pass over into a thin layer of sarcode, adjacent to the inner surface of the shell. The astropyle or the main-opening of the central capsule exhibits the usual radiate operculum and tubular proboscis of the PHÆODARIA (Pl. 115, fig. 3), and is directed towards the mouth of the shell. The number of the parapylæ or accessory openings seems to be variable in this family, and to correspond to the number of radial feet which arise from the shell. Therefore _Tuscaridium_ possesses only one parapyle, which is diametrically opposite to the mouth, lies on the aboral pole of the capsule, and is directed towards the single caudal tube. _Tuscarora_ seems to have three parapylæ, corresponding to the three radial feet, and _Tuscarusa_ probably has four parapylæ, directed towards its four radial feet; in the latter genus, however, the capsule was not observed (the shell being empty); and in the other Tuscarorida this important and difficult anatomical question must be solved by further accurate examinations.

The nucleus is nearly half as large as the central capsule, ellipsoidal, and contains numerous nucleoli. In one specimen of _Tuscarora belknapii_ I observed two nuclei in the central capsule, and in another specimen of the same species John Murray observed two central capsules (figured by him in the Narr. Chall. Exp., vol i. pl. A, fig. 15).

_Synopsis of the Genera of Tuscarorida._

Three equidistant aboral radial feet, 717. _Tuscarora_.
Four equidistant aboral radial feet, 718. _Tuscarusa_.
One single aboral foot or terminal spine, 719. _Tuscaridium_.

{1706}Genus 717. _Tuscarora_,[333] John Murray, 1876, _in schedulis_, Chall. Coll.

_Definition._--#Tuscarorida# with three radial aboral feet, and a variable number of circoral teeth.

The genus _Tuscarora_ comprises seven of the ten observed species of Tuscarorida, all seven agreeing in the possession of three perradial feet, which alternate originally (in four species), with three interradial teeth surrounding the mouth. The latter is armed in two other species with two teeth, and in one species with four teeth (compare above, p. 1704). The three perradial feet have a similar position as in the tripodal NASSELLARIA, so that they may be distinguished as an odd caudal foot and two paired lateral feet.

Subgenus 1. _Tuscarantha_, Haeckel.

_Definition._--Shell with three perradial equidistant feet and three interradial equidistant circoral teeth, alternating regularly with the former.

1. _Tuscarora bisternaria_, John Murray (Pl. 100, figs. 1, 1_a_).

_Tuscarora bisternaria_, John Murray, 1879, _in litteris_, Narr. Chall.
Exp., vol. i. p. 226, pl. A, figs. 16, 16_a._

Shell subspherical, with three lateral perradial feet in the equatorial zone, and three basal interradial teeth around the mouth, alternating regularly with the former. The three lateral feet arise either in the equator itself or a little above it, and are nearly horizontally expanded, descending a little towards the aboral pole. They are straight, cylindrical, twice to four times as long as the shell, geniculate at the inflated base, and covered with small thorns. A circle of six to eight basal pores in the base of each foot. The narrow mouth is triangular, surrounded by the thorny, inflated, subspherical bases of the three long, cylindrical, diverging, thorny teeth, which ascend obliquely and are longer than the shell; each tooth exhibits in the upper part of the inflated base a corona of eight to ten ovate holes.

_Dimensions._--Length of the shell 2.0, breadth 1.5.

_Habitat._--Central Pacific, Station 264, depth 3000 fathoms.

2. _Tuscarora murrayi_, n. sp. (Pl. 100, fig. 2). John Murray, 1876, L. N. 27, pl. 24, fig. 4.

Shell pear-shaped, with three circoral perradial feet in the upper third and three basal interradial teeth around the mouth. The three lateral feet are thin, cylindrical, arcuate, very bristly, arise immediately beyond the narrow tubular peristome, and ascend obliquely nearly to the height of the mouth; then they are curved downwards in a large arc, three to four times as long as {1707}the shell. The tubular peristome is about half as long as the shell, bottle-shaped, nearly three-sided prismatic, and bears three very long spinulate teeth, which in the lower half are parallel, in the upper slightly curved and diverging, at the base pear-shaped. Three ovate basal pores in the inflated base of each foot and each tooth.

_Dimensions._--Length of the shell 2.5, breadth 1.5.

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

3. _Tuscarora wyvillei_, n. sp. (Pl. 100, figs. 3, 3_a_-3_c_).

Shell subspherical, pellucid, thinner and more fragile than in the other species of the genus, with three aboral perradial feet in the lower third, and three interradial teeth around the wide mouth. The three feet are straight, conical, widely divergent, shorter than the shell, and arise from its lower third; on the inflated base of each four small ovate pores. The three teeth, alternating with them, are straight, cylindrical, arise from the margin of the mouth and diverge obliquely upwards. On the base of each foot four opposite cordate pores of very unequal size.

_Dimensions._--Length of the shell 1.5, breadth 1.4.

_Habitat._--South Pacific, Station 291, depth 2250 fathoms.

4. _Tuscarora tetrahedra_, John Murray (Pl. 100, figs. 4, 4_a_).

_Tuscarora tetrahedra_, John Murray, 1879, _in litteris_, Narr. Chall.
Exp., vol. i. p. 226, pl. A, fig. 19.

Shell tetrahedral or three-sided pyramidal, with three perradial basal feet, and three alternate, interradial, circoral teeth. The three rounded edges of the pyramid are prolonged directly over the aboral base into the three short, divergent, conical, basal feet, which are smooth and scarcely one-fourth as long as the shell. The base of each foot is pierced by four small crossed pedal pores. The narrow mouth, on the apex of the pyramid, is surrounded by three short and broad, triangular, spinulate teeth, each of which bears three slender, triangular, dental pores.

_Dimensions._--Length of the shell 2.5, breadth 2.0.

_Habitat._--Tropical Atlantic, Station 348, depth 2450 fathoms.

Subgenus 2. _Tuscaretta_, Haeckel.

_Definition._--Shell with three perradial equidistant feet, and with two circoral teeth, which are opposite in the radius of the odd dorsal foot; therefore a dorsal and a ventral tooth.

5. _Tuscarora tubulosa_, John Murray (Pl. 100, figs. 5, 5_a_, 5_b_).

_Tuscarora tubulosa_, John Murray, 1879, _in litteris_, Narr. Chall.
Exp., vol. i. p. 226, pl. A, fig. 17.

Shell subspherical, with three lateral perradial feet on the upper half of the shell (above the equator), and two parallel vertical teeth on the mouth. The three feet arise between the upper and {1708}the middle third of the shell, diverge upwards, and are slightly arcuate, covered with numerous curved spines. Their base is pierced by four pedal pores. The peristome is a narrow sagittal fissure, placed in the radius of the odd or dorsal foot, and bears two opposite teeth on its two corners. The teeth are cylindrical, longer than the shell, covered with curved spines, and ascend in a parallel and vertical manner; each is perforated at the base by three large dental pores.

_Dimensions._--Length of the shell 1.4, breadth 1.2.

_Habitat._--North Pacific, Stations 244 to 250, depth 2050 to 3050 fathoms.

6. _Tuscarora porcellana_, John Murray (Pl. 100, figs. 6_a_, 6_b_).

_Tuscarora porcellana_, John Murray, 1819, _in litteris_, Narr. Chall.
Exp., pl. A, fig. 18.

Shell pear-shaped, with three lateral perradial feet in the lower half of the shell, and two parallel vertical teeth on the mouth. The three lateral feet arise between the middle and the lower third of the shell, are straight, cylindrical, spinulate, about as long as the shell and diverge downwards; their base is pierced by four ovate pedal pores. The two parallel and vertical teeth are straight, shorter than half the shell, and opposite in the sagittal plane, on the two corners of the narrow mouth, in the radius of the dorsal odd foot. The base of each tooth exhibits four cordate dental pores.

_Dimensions._--Length of the shell 1.5, breadth 1.3.

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

Subgenus 3. _Tuscarilla_, Haeckel.

_Definition._--Shell with three perradial equidistant feet, and with four crossed equidistant teeth around the mouth.

7. _Tuscarora belknapii_, John Murray.

_Tuscarora belknapii_, John Murray, 1879, _in litteris_, Narr. Chall.
Exp., vol. i. p. 226, pl. A, figs. 15, 15_a_-15_d._

Shell pear-shaped, with three lateral perradial feet in the upper third, and four crossed teeth around the mouth. The three arcuate feet are thin, cylindrical, covered with small curved thorns, and arise at the base of the peristome, between the upper and middle third of the shell; they ascend diverging to the height of the mouth, and are then curved downwards in a large arc, twice to three times as long as the shell. The base of each foot is dilated and pierced by three pedal pores. The four thin and long teeth of the peristome are similar to the feet, ascend in a slightly diverging manner, and are so placed that two opposite lie in the sagittal plane (or in the radius of the dorsal odd foot), whilst the two others are opposite in the frontal plane (perpendicular to the former). The base of each tooth exhibits two large ovate dental pores.

_Dimensions._--Length of the shell 2.5, breadth 1.5.

_Habitat._--South Pacific, Station 293, depth 2025 fathoms.

{1709}Genus 718. _Tuscarusa_,[334] n. gen.

_Definition._--#Tuscarorida# with four radial aboral feet and a variable number of circoral teeth.

The genus _Tuscarusa_ differs from the preceding closely allied _Tuscarora_ in the possession of four radial feet instead of three. The mouth of the single observed species is a narrow sagittal fissure, and armed with two opposite teeth (a dorsal and a ventral, as in the subgenus _Tuscaretta_). The four lateral feet form a cross, and lie opposite in pairs, in two diagonals of the square, which is bisected by the sagittal mouth.

1. _Tuscarusa medusa_, n. sp. (Pl. 100, figs. 7, 7_a_).

Shell ovate, with four crossed lateral feet in the upper third, and with two opposite teeth around the mouth. The four feet are opposite in pairs in two diameters of the shell, perpendicular one to another, and arise between the upper and middle third of the shell; they are thin, cylindrical, spinulate, arcuate, and ascend to the height of the mouth diverging upwards; then they are curved downwards in a large arc. They lie in the diagonals of the square, which is halved by the narrow and long fissure of the mouth. This lies in the original sagittal plane, and from its two corners arise the two opposite teeth (a dorsal and a ventral), diverging upwards, cylindrical and slightly curved. The base of each foot is pierced by four pedal pores, of each tooth by two dental pores.

_Dimensions._--Length of the shell 1.2, breadth 1.0.

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

Genus 719. _Tuscaridium_,[335] n. gen.

_Definition._--#Tuscarorida# without radial aboral feet, but with a terminal axial caudal foot, and a variable number of circoral teeth.

The genus _Tuscaridium_, comprising two closely allied species, differs from the two preceding genera of Tuscarorida in the absence of lateral radial feet, which are represented by a single large caudal spine placed in the main axis of the shell, at its aboral pole. The shell is therefore spindle-shaped, and not ovate or subspherical, as in the two other genera. It reaches in the two observed species a length of more than 3 mm.

1. _Tuscaridium cygneum_, Haeckel.

_Tuscarora cygnea_, John Murray, 1879, in litteris, Narr. Chall. Exp.,
vol. i. p. 226, pl. A, fig. 20.

Shell spindle-shaped, twice as long as broad (in the transverse section circular), equally tapering towards both poles of the main axis. The aboral pole bears a thin, cylindrical, straight, caudal {1710}spine, placed in the prolongation of the main axis, and about half as long as the shell. Its base is pierced by two large opposite pedal pores. The anterior or oral pole bears a cylindrical peristome, similar to a bird's head, and curved towards the ventral face; on both sides of its neck (at right and left) a series of three or four irregular, ovate, buccal holes. The neck bears four cylindrical, spinulate, radial tubes (two on each side), crossed nearly horizontally, and placed in two diagonal planes perpendicular one to another; these planes correspond to those in which the four feet of _Tuscarusa medusa_ lie. The base of each tube is pierced by four dental pores.

_Dimensions._--Length of the shell 3.2, breadth 1.6.

_Habitat._--North Pacific, Station 250, depth 3050 fathoms.

2. _Tuscaridium lithornithium_, n. sp. (Pl. 100, figs. 8, 8_a_, 8_b_).

Shell spindle-shaped, twice as long as broad, very similar to the preceding closely allied species. It differs from the latter in the following characters:--The curved proboscis of the peristome is broader and more spiny. The four radial tubes of the mouth and the terminal caudal spine are very spiny (in the preceding species nearly smooth). The number of buccal holes (six to eight on each side of the mouth) is twice as great as in the former. Each circoral tube is pierced at the base by six or eight (in the former by four pores), and the base of the caudal spine exhibits a cross of four pores (in _Tuscaridium cygneum_ only two pores).

_Dimensions._--Length of the shell 3.6, breadth 1.8.

_Habitat._--North Pacific, Station 264, depth 3000 fathoms.

----

Order IV. PHÆOCONCHIA, Haeckel, 1879.

_Definition._--PHÆODARIA with a bivalved lattice-shell, composed of two free opposite valves (a dorsal and a ventral), between which the central capsule is enclosed.

Family LXXXIII. #CONCHARIDA#, Haeckel, 1879 (Pls. 123-125).

_Concharida_, Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec.
12, p. 6.

_Definition._--PHÆODARIA with a bivalved lattice-shell, which is spherical or lenticular, and composed of two equal or unequal boat-shaped valves, a dorsal and a ventral. The valves bear neither an apical latticed cupola or galea, nor hollow radial tubes. The central capsule is placed in the aboral half of the shell-cavity, and so enclosed between both valves, that its three openings lie in the open frontal fissure between them (the astropyle on the oral pole of the main axis, the two parapylæ on both sides of its aboral pole, at right and left).

{1711}The family #Concharida# and the two following closely allied families, the Coelodendrida and Coelographida, compose together the most remarkable and interesting suborder of #Phæoconchia# (or "PHÆODARIA bivalva"), differing from all the other Radiolaria in the possession of a bivalved lattice-shell, composed of two separate valves, like the shell of a Brachiopod. The central capsule is so enclosed between the two fenestrated valves that its three openings lie in the horizontal open (frontal) fissure between them, the astropyle or main-opening on the oral pole of the main axis; the two secondary openings or parapylæ on the two sides of its aboral pole, at right and left. The plane in which the three openings lie is therefore the frontal plane, dividing the entire body into a dorsal and a ventral half. The two valves, accordingly, must be considered as dorsal and ventral valves (as in the Brachiopoda), and the symmetrical halves of each valve as right and left. These halves may be always easily distinguished, since the oral pole of each valve is constantly different from the aboral pole. The voluminous phæodium always lies in the oral half, and the central capsule in the aboral half of the shell-cavity, whilst the calymma encloses the whole shell.

The Concharida differ from the other two families of bivalved PHÆODARIA in the absence of the apical galeas, and the branched hollow tubes arising from them. Each of these two cupolas, which are at the opposite poles of the sagittal axis (one cupola on the apex of each valve), is in the Coelographida connected by a simple or double frenulum with a peculiar rhinocanna, or an open nasal tube directed towards the mouth; whilst the cupolas of the Coelodendrida possess neither a rhinocanna nor a frenulum. The three families of #Phæoconchia# may therefore represent a phylogenetical series, the common root of which are the Concharida. From these are developed the Coelodendrida by development of an apical cupola or galea on each valve, and of hollow radial tubes arising from it; whilst the Coelographida are developed from the latter by production of a rhinocanna on the base of each cupola, and of one or two frenula connecting the former with the latter.

All the Concharida described in the following pages (seven genera and thirty species), are perfectly new to science, and not a single form of this interesting family was known before the explorations of the Challenger. Some species (mainly of the genera _Conchidium_ and _Conchopsis_) are by no means rare, and are found in great numbers at some stations of the tropical seas (in the Pacific as well as in the Atlantic). All described species are closely allied, agree in the majority of characters, and are easy to distinguish from all the other Radiolaria. Some few forms of Concharida, however, form a direct passage to the Coelodendrida.

Regarding the probable origin of the Concharida (and therefore also of all other #Phæoconchia# derived from the latter), two different hypotheses are possible. They have either been derived directly from the skeletonless _Phæodina_, by development of a bivalved lattice-shell; or they may be derived from PHÆODARIA with a simple {1712}spherical lattice-shell (Castanellida), by the halving of this latter, or its splitting into two hemispherical valves; the former hypothesis is more probable than the latter.

The two valves of the lattice-shell (Pls. 123-126) must in the Concharida (as in all other #Phæoconchia#) be distinguished as dorsal and ventral, and may therefore be compared with the two valves of the Brachiopoda, not with those of the Lamellibranchia. This important morphological distinction is expressed by the constant position of the central capsule within the shell-cavity. The capsule always exhibits the character of the "TRIPYLEA" and has three tubular openings, placed in the frontal or lateral plane of the unicellular body. In the same plane lies the open frontal fissure between the two valves, and the three openings are so disposed in it that the large anterior main-opening (or the astropyle) is placed on the oral pole of the main axis, whilst the two accessory small lateral openings or parapylæ are placed on both sides of the aboral pole, at the right and left. Therefore in a dorsal or ventral view all three openings are visible (Pl. 123, figs. 1, 8_a_); in the usual lateral view, however, from the right or left side, only two openings are visible, the astropyle on the anterior, and one parapyle (right or left) near the posterior pole of the main axis (Pl. 123, figs. 8, 9; Pl. 124, figs. 6, 10). The posterior view (from the aboral pole) shows the two parapylæ, at right and left; in the anterior view (from the oral pole) the astropyle may be visible, but usually it is completely hidden in the dark voluminous phæodium. This latter envelops sometimes nearly the whole capsule as an opaque conglomeration of green or brown phæodella (Pl. 123, figs. 8, 9); but usually the phæodium fills up the anterior (oral) half of the shell-cavity, whilst the capsule occupies the posterior (aboral) half (Pl. 124, figs. 6, 10).

The dorsal shell-valve is in almost all PHÆODARIA smaller or somewhat different in shape from the ventral valve, and this difference is often very striking (Pl. 124, figs. 3-16); but in a few species both valves are so similar, that I could not discover any certain difference. This equality of the two valves occurs mainly in those Concharida which pass over into the Coelodendrida; in these latter as well as in the Coelographida, both valves are usually equal in size and form. Whilst the main axis (or the longitudinal axis of the body) in the two latter families of #Phæoconchia# seems to be normally vertical (in the living and freely floating body), in the living Concharida it is probably horizontal, so that the larger and heavier ventral valve lies below the smaller and lighter dorsal valve.

The geometrical fundamental form of the body is therefore in the Concharida dipleural or bilaterally symmetrical, and we distinguish in it the same three dimensive axes, as in all other dipleural forms. On the anterior or oral pole of the main axis (or longitudinal axis) lies the mouth of the shell, and behind it the phæodium; on the opposite posterior or aboral pole lies the hinge of the shell (comparable to the shell-hinge of the Brachiopoda) and in front of it the central capsule. The sagittal (or {1713}dorso-ventral) axis, determining the height of the shell, has on its dorsal (or upper) pole the apex or highest point of the dorsal valve, on its ventral (or lower) pole the apex or lowest point of the ventral valve. The two poles of the frontal (lateral or transverse) axis are equal and are determined by the two parapylæ of the capsule, and the corresponding points of the shell-fissure between both valves. Usually the main-axis is the longest, the frontal axis the shortest, and between both the sagittal axis.

In regard to the three dimensive planes which are determined by these three axes, perpendicular to one another, they are rarely of nearly equal size (as in some subspherical species), usually the sagittal plane (separating the right and left halves of the body) is the largest; the cinctural or equatorial plane (separating oral and aboral halves) is the smallest, and the frontal or lateral plane (separating dorsal and ventral halves) is intermediate in size. The relation of the three perimeters of these three planes corresponds to that proportion; the sagittal perimeter (in which the keel of the compressed valves lies) is the largest; the cinctural or equatorial perimeter (separating the anterior phæodium and the posterior central capsule) is the smallest, and the frontal or lateral perimeter (in which the fissure between the valves lies) is intermediate in size.

The general form of the single valves is very varied in the different species, in the majority boat-shaped or hat-shaped, more or less laterally compressed, in a few forms hemispherical. In _Conchopsis_ (Pl. 125) and _Conchoceras_ (Pl. 124, figs. 15, 16) the lateral parts of the valves (right and left) are vaulted, whilst their median parts are so strongly compressed that they form a sharp sagittal keel, and then the shell in the dorsal or ventral view appears spindle-shaped (Pl. 123, fig. 8_a_; Pl. 125, fig. 8). Often the frontal margins of the valves are somewhat constricted (Pl. 124, fig. 7).

The junction between the two valves of the shell is always loose, but not so loose as in the two following families. In the Coelodendrida and Coelographida the two valves are either perfectly free and separated by a frontal zone of jelly, or in very loose contact on the frontal margins. In the Concharida, however, the margins of both valves seem to be usually in contact, and their connection is effected in a double way. In the subfamily Conchasmida (comprising the genera _Concharium_ and _Conchasma_, Pl. 123, figs. 1-6) the lateral margins of both valves are smooth, not dentated, and fit one into another like the two parts of a box, or like the two valves of a Diatom (_Navicula_). In the second subfamily, however, Conchopsida (comprising the five other genera, Pls. 124, 125), the lateral margins of the valves are dentate, usually provided with a series of numerous strong conical teeth, and the teeth of both valves so catch into one another, that their union is rather firm (like the margin of the shells of _Tridacna_, _Pecten_, and other Lamellibranchiata). Besides, a more solid junction is often effected on the posterior or aboral part of the margins, which we shall call the hinge. Here often peculiar strong teeth catch one into another, and in the majority of species {1714}two aboral spines are developed, the caudal horns (a dorsal and a ventral); these are very large in _Conchoceras_ (Pl. 124, figs. 15, 16). But a peculiar and most interesting kind of junction is effected in some Concharida by a true ligament between the valves (Pl. 123, figs. 8, 9; Pl. 125, fig. 2). This ligament is always placed on the aboral hinge, is of dark brown colour, and is not dissolved by mineral acids unless long applied. It may preserve the connection of the posterior parts of both valves, when their anterior parts are removed one from another, just as in the Brachiopoda. I observed this interesting ligament mainly in the genus _Conchopsis_, but not in all species, and it is not yet certain whether it is a constant organ in these and some other Concharida.

In the majority of Concharida the lateral margins of the two valves project slightly inwards into the cavity, and in some species of _Conchopsis_ these inner borders are so broadened that they form a broad, horizontal, fenestrated inner shelf, comparable to the deck of a boat or to the velum of the Hydromedusæ or Craspedotæ (Pl. 125, fig. 9). In this case the velum surrounds the ovate aperture through which the two lobes of the central capsule (dorsal and ventral) enter into the cavity of both valves.

The mouth of the shell lies on the oral pole of the main axis, and is therefore opposed to the aboral hinge. The two valves are here usually more or less emarginate, so as to form a transverse mouth with an upper and a lower lip (Pl. 124, figs. 6, 7, 11). The form of these two lips is often very different and characteristic of particular species (Pl. 124, figs. 3, 15, 16). The mouth remains in many species constantly open, even when the frontal fissure is closed (figs. 7, 16). Since the centre of the shell mouth lies in the prolongation of the proboscis arising from the operculum of the central capsule, probably the main stream of sarcode, issuing from the latter, becomes protruded by the former.

Apophyses of the shell (besides the teeth of the margins) are completely wanting in three genera, _Concharium_, _Conchellium_, and _Conchopsis_ (Pl. 123, figs. 1-4, 7; Pl. 125). The four other genera possess free apophyses or spines, which we call horns. They are probably important as the beginnings of those large hollow tubes which are characteristic of the two following families, Coelodendrida and Coelographida. We distinguish two different forms of horns, apical horns on the poles of the sagittal axis, and caudal horns on the aboral pole of the main axis; the former probably correspond to the sagittal tubes and the latter to the caudal tubes of the two following families. Apical horns are found in a single genus only, _Conchonia_ (Pl. 124, figs. 10-14). Here either on one pole or on poles of the sagittal axis a horn is developed, usually curved backwards. Sometimes the base of this conical horn is inflated and fenestrated, and may represent the beginning of the formation of the galea or apical cupola of the Coelodendrida.

The two caudal horns are opposite on the aboral hinge of the shell, one arising from the posterior end of each valve. Usually they are short and thick, pyramidal, the {1715}ventral horn larger than the dorsal (Pl. 124, figs. 3, 6). Rarely the two caudal horns are fenestrated at the base and reach a considerable size, as in _Conchoceras_ (Pl. 124, figs. 15, 16).

The walls of the bivalved shell usually exhibit in the Concharida a rather solid shape and regular structure, with an elegant network of regularly arranged pores. But in some species the walls of the shell become very thin and fragile, and assume the same shape (with very irregular network), as in the Coelodendrida and Coelographida. The pores are usually small and numerous, circular, often hexagonally framed (Pl. 125, figs. 4-6). They pierce the thick shell-wall either in a radial or in an oblique direction. Sometimes each pore is armed with six radial teeth (Pl. 123, fig. 7_a_). At other times each pore represents an oblique ampullaceous canal, dilated in its middle part, with two narrow openings (Pl. 125, figs. 5_a_, _b_, _c_, 6). The pores are so arranged in the majority of species that they form regular curved series, which are separated by prominent crests, and converge towards the poles of the main axis. Usually the marginal pores (along the frontal margin of the valves) are much smaller (compare Pls. 123-125).

The _central capsule_ of the Concharida, very well preserved in numerous specimens of the Challenger collection, constantly possesses the same situation and structure. It is always enclosed in the aboral or posterior half of the shell-cavity, whilst the oral or anterior half is filled up by the phæodium. The free spaces between both and between the inner surface of the shell are completely filled up by the jelly of the calymma, which also covers the whole shell as a thin outer jelly-envelope. The form of the central capsule is sometimes nearly spherical, usually somewhat compressed in the direction of the main axis, and sometimes also in the direction of the frontal axis (Pl. 123, figs. 1-9). In some species it becomes bilobed, with an upper dorsal and a lower ventral lobe, and in some others it becomes triangular (Pl. 125, fig. 7). Its two membranes (inner and outer) are often separated by a broad colourless interval, containing a clear fluid or jelly (Pl. 123, figs. 8, 9). The nucleus is usually about half as large as the central capsule and ellipsoidal, its longer axis lying in the sagittal diameter of the body. Several specimens (of different genera) contained two separate nuclei, one placed in the dorsal, the other in the ventral half of the capsule (Pl. 124, fig. 6). This duplication of the nucleus is probably the preparation for the division of the capsule. The division will be probably effected in the frontal plane, so that each half of the bisected capsule gets one nucleus and one valve, and the other valve becomes newly formed (in a way similar to that in the bivalved Diatomaceæ). The astropyle, or the main-opening of the capsule, is closed by a radiate operculum, from which arises a tubular proboscis; this lies in the main axis of the body, is directed towards the anterior mouth of the shell, and surrounded by the phæodium. The two shorter tubes of the paired parapylæ, or the accessory lateral openings, lie on the posterior or caudal side of {1716}the capsule, at right and left, and are directed half backwards, half outwards (towards the frontal fissure between the valves, Pl. 123, figs. 1, 8_a_).

The phæodium exhibits in all Concharida the same characteristic shape, and represents a dark conglomeration of phæodellæ, filling up the anterior or oral half of the shell-cavity. Usually it is bilobed, divided into a dorsal and a ventral lobe or wing, which fills up the corresponding valve of the shell (Pl. 123, figs. 8, 9). The phæodium is commonly more voluminous than the capsule, and surrounds its anterior half, more rarely it encloses nearly the entire capsule (Pl. 124, figs. 6, 10). Its colour is usually olive, sometimes more greenish, at other times more brownish, in some species nearly black. The phæodellæ, or the roundish granules which compose the phæodium, exhibit the same shape as in all other PHÆODARIA (compare above, p. 1535). Sometimes peculiar rather oblong nucleated cells are scattered in great numbers between the phæodellæ, probably parasites or symbiontes (Pl. 123, figs. 7-9, 9_a_).

_Synopsis of the Genera of Concharida._

-------------------------------------------------------------------------
I. Subfamily Conchasmida.
Lateral edges of the two valves smooth, without teeth.
-------------------------------------------------------------------------
{ Aboral hinge without
Valves without sagittal { horns, 720. _Concharium_.
keel, nearly hemispherical {
or slightly compressed. { Aboral hinge with two
{ horns (one on each
{ valve), 721. _Conchasma_.
-------------------------------------------------------------------------
II. Subfamily Conchopsida.
Lateral edges of the two valves dentate, with a series of prominent
teeth on both sides. The teeth of both valves catch one into another.
-------------------------------------------------------------------------
{ Aboral hinge without
{ horns, 722. _Conchellium_.
{
Valves without sagittal { Aboral hinge with two
keel, nearly hemispherical { horns. No apical horn, 723. _Conchidium_.
or slightly compressed. {
{ Aboral hinge with two
{ horns. Apex also with
{ a horn, 724. _Conchonia_.

{ Aboral hinge without
Valves with a sharp { horns, 725. _Conchopsis_.
sagittal keel, strongly {
compressed on both sides, { Aboral hinge with two
boat-shaped. { horns (one on each
{ valve). 726. _Conchoceras_.

Subfamily 1. CONCHASMIDA, Haeckel.

_Definition._--#Concharida# with the lateral margins of the two valves smooth, without interlocking teeth.

Genus 720. _Concharium_,[336] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6.

_Definition._--#Concharida# with the lateral margins of the valves smooth, without sagittal keel and without horns on the hinge.

{1717}The genus _Concharium_ is the simplest and the most primitive form of all Concharida; it may be regarded as the common ancestral form of the whole family. The entire shell is usually almost spherical, without horns or teeth, and may be regarded as a _Castanella_ which is bisected or broken into two equal hemispherical halves. The lateral margins of the two hemispherical valves are smooth, without teeth, and catch one into the other like the two valves of a Diatom, or the two halves of a bivalved box. _Concharium_ agrees in this simple shape of the frontal margins with the following genus _Conchasma_, and represents with it the small subfamily Conchasmida.

1. _Concharium bivalvum_, n. sp. (Pl. 123, figs. 2, 2_a_).

Shell spherical, smooth. Diameter in all directions nearly the same. Borders of the two hemispherical valves circular, smooth, about twice as broad as the pores. In the half frontal perimeter of the shell (along the right and the left border of each valve) twenty-two to twenty-four pores, in the half sagittal perimeter (in the middle line of each valve) eighteen to twenty-two pores, in the half equator (in the cinctural perimeter of each valve) twenty to twenty-two pores. All pores circular, of the same size, twice as broad as their bars.

_Dimensions._--Length of the shell (longitudinal diameter) 0.35, height (sagittal diameter) 0.34, breadth (lateral diameter) 0.33.

_Habitat._--North Atlantic, west of Madeira, Station 354, depth 1675 fathoms.

2. _Concharium nucula_, n. sp. (Pl. 123, fig. 3).

Shell pear-shaped, with costate surface. Oral face somewhat truncated, broader than the aboral face. Its longitudinal diameter about one-fifth longer than the two other diameters. Borders of the two valves ovate, smooth, about as broad as the pores. In the half frontal perimeter of the shell twenty-two to twenty-four pores, in the half sagittal perimeter eighteen to twenty, in the half equator sixteen to eighteen. Pores irregularly roundish, three to four times as broad as the bars. The pores are so disposed in meridional rows that the crests between the rows converge towards the two poles of the sagittal axis.

_Dimensions._--Length of the shell 0.2, height 0.18, breadth 0.16.

_Habitat._--South Atlantic (west of Tristan da Cunha), Station 332, depth 2200 fathoms.

3. _Concharium diatomeum_, n. sp. (Pl. 123, fig. 1).

Shell nearly spherical, slightly lenticular, somewhat compressed in dorso-ventral direction; the sagittal diameter therefore somewhat shorter than the two others. Borders of the two hemispherical valves nearly circular, quite smooth, about as broad as the length of the largest pores. In the half frontal perimeter of the shell forty-four to fifty pores; in the half sagittal perimeter twenty to twenty-four; in the half equator thirty to thirty-three. Pores different in form and size; the {1718}marginal pores small, nearly circular; the dorsal and ventral pores oblongish-hexagonal, twice as long as broad, about four to six times as long as the bars, regularly arranged in transverse rows.

_Dimensions._--Length of the shell 0.22, height 0.21, breadth 0.2.

_Habitat._--Tropical Atlantic, near Sierra Leone, Station 348, depth 2450 fathoms.

4. _Concharium bacillarium_, n. sp. (Pl. 123, fig. 4).

Shell walnut-shaped, with panelled surface; oral and aboral face of the same form. Its longitudinal diameter about one-fifth longer than the two other diameters. Borders of the two cup-shaped valves elliptical, smooth, with a prominent edge, about as broad as the larger pores. In the half frontal perimeter of the shell fifty to fifty-five pores, in the half sagittal perimeter thirty-six to forty, in the half equator also thirty to forty. Pores hexagonally framed. The pores are tapering in size from the sagittal plane towards the valve-margins, and so regularly arranged in meridional rows that the crests between the latter converge towards both poles of the longitudinal axis.

_Dimensions._--Length of the shell 0.2, height 0.15, breadth 0.15.

_Habitat._--Tropical Atlantic, off St. Helena, Station 340, depth 1500 fathoms.

5. _Concharium fragilissimum_, n. sp.

Shell subspherical, very thin-walled and fragile. Diameter in all directions nearly the same. Oral and aboral face scarcely different. Margins of the hemispherical valves extremely thin and hyaline. Pores irregularly roundish, of very different sizes and unequal forms. The fragile shell of this species differs in general shape from that of all other Concharida, and is like that of the Coelodendrida (Pl. 121, fig. 3), but exhibits neither an apical cupola or galea, nor radial tubes arising from it. It may be perhaps a young specimen of _Coelodendrum_.

_Dimensions._--Diameter of the shell 0.22, of the pores 0.002 to 0.02.

_Habitat._--Mediterranean, Portofino (Haeckel), surface.

Genus 721. _Conchasma_,[337] n. gen.

_Definition._--#Concharida# with the lateral margins of the valves smooth, without sagittal keel, but with two caudal horns on the hinge (a dorsal and a ventral).

The genus _Conchasma_ is closely allied to the preceding _Concharium_, and has the same hemispherical valves with smooth margins, without teeth; but it differs from the latter in the development of two caudal horns or posterior spines on the aboral hinge, one horn on the aboral end of each valve. The three species of this genus were all found in great depths of the Antarctic Ocean, in Diatom ooze, between 1260 and 1975 fathoms, at Stations 152 to 157.

{1719}1. _Conchasma radiolites_, n. sp. (Pl. 123, fig. 5).

Shell nearly spherical, somewhat compressed on both sides; the dorsal valve smaller, flatter and shorter than the ventral valve. In the half sagittal perimeter of the shell twenty to twenty-two pores, in the half frontal perimeter twelve to fourteen, in the half equator eighteen to twenty. All pores nearly of the same size, circular, hexagonally framed, scarcely as broad as the bars. The two horns of the hinge are four-sided pyramidal, of different sizes; the ventral horn (of the larger valve) two to three times as long as the dorsal horn (of the smaller valve); the latter twice as long as a pore.

_Dimensions._--Length of the shell 0.16, height 0.15, breadth 0.14.

_Habitat._--Antarctic Ocean, Station 154, depth 1800 fathoms.

2. _Conchasma sphærulites_, n. sp. (Pl. 123, fig. 6).

Shell nearly spherical, somewhat compressed on both sides, the frontal diameter therefore somewhat shorter than the two others. Both valves nearly of the same size, hemispherical, their borders smooth, twice as broad as the largest pores. In the half sagittal perimeter of the shell twenty-eight to thirty pores, in the half frontal perimeter twenty to twenty-two, in the half equator twenty-four to twenty-six. Size of the pores increasing from the borders towards the top of the valves. One series of very small pores along the frontal free margin of each valve. Pores roundish-polygonal, three to four times as broad as the bars. The two horns of the hinge are of equal size, four-sided pyramidal, and twice as long as the larger pores.

_Dimensions._--Length of the shell 0.18, height 0.18, breadth 0.16.

_Habitat._--Antarctic Ocean, Station 152, depth 1260 fathoms.

3. _Conchasma hippurites_, n. sp.

Shell nearly spherical, scarcely compressed. The frontal diameter equal to the two others. Both valves equal. In the half sagittal perimeter of the shell thirty-two to thirty-four pores, in the half frontal perimeter twenty-four to twenty-six, in the half equator twenty-six to twenty-eight. All pores of nearly equal size, circular, polygonally framed, twice as broad as the bars. The two horns of the hinge are large, three-sided pyramidal, the ventral horn twice as long as the dorsal, and four to six times as long as one pore.

_Dimensions._--Length of the shell 0.22, height 0.21, breadth 0.2.

_Habitat._--Antarctic Ocean, Station 157, depth 1950 fathoms.

Subfamily 2. CONCHOPSIDA, Haeckel.

_Definition._--#Concharida# with the lateral margins of the two valves dentate, the teeth of both catch one into another.

{1720}Genus 722. _Conchellium_,[338] n. gen.

_Definition._--#Concharida# with the lateral margins of the valves dentate, without sagittal keel and without horns on the hinge.

The genus _Conchellium_ and the four following genera represent together the subfamily Conchopsida, differing from the Conchasmida in the dentate lateral margins of the two valves. These are armed with a series of strong, conical teeth, and catch one into another just as the two valves of many Lamellibranchiata and Brachiopoda do (Pl. 124, figs. 1-16). _Conchellium_ is the simplest form among the Conchopsida, since the valves are hemispherical, and possess neither a sagittal keel nor projecting horns.

1. _Conchellium tridacna_, n. sp. (Pl. 123, figs. 7, 7_a_).

Shell nearly spherical, finely tuberculated, the sagittal diameter somewhat longer than the two others. Borders of the two hemispherical valves smooth in 0.2 of the oral, and 0.1 at the aboral part, dentated in the remaining 0.7 part; on one side of each valve fourteen to sixteen very strong and long teeth, all nearly of the same size, about one-fourth as long as the shell-radius. In the half frontal perimeter of the shell (on one border of each valve) twenty-five to thirty pores, in the half sagittal perimeter thirty-five to forty, in the equator twenty-five to thirty. Pores circular, hexagonally framed, of equal size (except some smaller rows along the fissure), twice as broad as the bars. On the conical inside of each funnel-like pore six small spinules, between every three neighbouring pores a triangular facette (fig. 7_a_).

_Dimensions._--Length of the shell 0.34 to 0.38, height 0.38 to 0.42, breadth 0.32 to 0.36.

_Habitat._--North Pacific, Stations 250 to 253, depth 2740 to 3125 fathoms.

2. _Conchellium hippopus_, n. sp.

Shell nearly spherical, in the lateral perimeter (along the girdle-fissure) somewhat constricted. Borders of the two hemispherical valves semicircular, smooth in 0.3 of the oral, and 0.2 of the aboral part, dentated only in the remaining 0.5 middle part; on one side of each valve seven to eight very strong and long teeth, increasing in size towards the mouth, the longest (foremost) teeth nearly one-third as long as the shell-radius. In the half frontal perimeter of the shell (along one border of each valve) twenty-four to twenty-eight pores, in the half sagittal perimeter thirty-two to thirty-six, in the half equator twelve to fourteen. Pores circular, twice to three times as broad as the bars, smaller along the fissure.

_Dimensions._--Length of the shell 0.06, height 0.065, breadth 0.055.

_Habitat._--Central area of the Tropical Pacific, Station 274, depth 2750 fathoms.

{1721}Genus 723. _Conchidium_,[339] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6.

_Definition._--#Concharida# with the lateral margins of the valves dentate, without sagittal keel and apical horns, but with two caudal horns on the hinge (a dorsal and a ventral).

The genus _Conchidium_ is the most common form of all Concharida, and some of its species occur in great numbers in the tropical zone of the Pacific and the Atlantic, on the surface as well as at various depths. It differs from the preceding _Conchellium_, its ancestral form, in the development of two caudal horns, or two strong pyramidal spines which arise from the posterior end of the valves; the dorsal horn usually is smaller than the ventral.

1. _Conchidium terebratula_, n. sp. (Pl. 124, figs. 1, 2).

Shell subspherical, smooth; both valves of nearly equal size and form, hemispherical. The three dimensive axes of the body are almost equal. Margins of the valves dentate in nearly the whole periphery; on each side of one valve eleven or twelve strong conical teeth, all of the same size. Aboral hinge with two short and stout four-sided pyramidal horns of equal length. Mouth with two equal short lips. Pores of the shell subregular, circular, three to four times as broad as the bars, in the dorsal valve twice as large as in the ventral valve.

_Dimensions._--Diameter of the shell 0.24 to 0.28.

_Habitat._--Central Pacific, Stations 270 to 274, surface, and at various depths.

2. _Conchidium thecidium_, n. sp. (Pl. 124, fig. 6).

Shell subspherical, slightly compressed on both sides. Dorsal valve somewhat smaller than the ventral, of similar form. Principal axis of the shell somewhat longer than the sagittal, and this longer than the frontal axis. Margins of the valves smooth in the oral quarter, strongly dentate in the remainder; on each side of one valve eight or nine very large triangular teeth, half as long as the height of the valve. Aboral hinge with two unequal, stout, four-sided pyramidal horns; the dorsal horn half as long at the ventral. Mouth with two unequal lips, the upper shorter than the lower. Pores of the shell subregular, hexagonal, three to four times as broad as the bars. The figured specimen, well preserved, contained in the central capsule two nuclei, one in the dorsal, the other in the ventral half.

_Dimensions._--Length of the shell 0.28 to 0.33, height 0.27 to 0.3, breadth 0.22 to 0.24.

_Habitat._--South Pacific, Station 292, depth 1600 fathoms.

{1722}3. _Conchidium rhynchonella_, n. sp. (Pl. 124, fig. 3).

Shell with three different dimensive axes in the proportion = 6 : 5 : 4. Ventral valve semi-ovate, larger than the humpbacked dorsal valve. Margins of the valves dentate almost in the whole periphery; on each side of one valve twelve or thirteen strong conical teeth. Aboral hinge with two very unequal horns, the dorsal much smaller than the pyramidal ventral. Under lip of the mouth emarginate, much larger than the acute upper lip. Pores subregular, circular, twice as broad as the bars.

_Dimensions._--Length of the shell 0.3, height 0.25, breadth 0.2.

_Habitat._--North Pacific, Stations 244 to 253, surface, and at various depths.

4. _Conchidium dimerella_, n. sp.

Shell laterally compressed, very similar to the preceding, but differing in the following characters:--Proportion of the three axes = 7 : 5 : 4. Ventral valve hemispherical, larger than the humpbacked dorsal. Margins of the valves dentate in the middle half only, whilst the anterior and posterior quarters are smooth; on each side of one valve six or seven strong conical teeth. Ventral horn of the aboral hinge very large, pyramidal, one-third as long as the shell and three times as long as the dorsal.

_Dimensions._--Length of the shell 0.35, height 0.25, breadth 0.2.

_Habitat._--North Pacific, Station 236, surface.

5. _Conchidium leptæna_, n. sp. (Pl. 124, figs. 4, 5).

Shell laterally compressed, with three different dimensive axes of the proportions = 4 : 3 : 2. Ventral valve larger than the dorsal, both semi-ovate. Margins of the valves dentate along the lateral sides; on each side of one valve nine or ten conical teeth, the middle of which are smaller. Aboral hinge with two unequal short horns; the dorsal rudimentary. Under lip of the mouth pointed, much larger than the truncate upper lip. Pores circular, about twice as broad as the bars.

_Dimensions._--Length of the shell 0.2, height 0.15, breadth 0.1.

_Habitat._--Central Pacific, Stations 271 and 272, surface, and at various depths.

6. _Conchidium argiope_, n. sp. (Pl. 124, figs. 7-9).

Shell laterally compressed, with three different dimensive axes of the proportion = 6 : 5 : 3. Both valves nearly equal. Margins of the shell dentate along the lateral fissure, on each side of one valve thirteen or fourteen thin conical teeth. Aboral hinge with two equal, short, pyramidal horns. Mouth with two equal short lips. Pores subregular, circular, twice as broad as the bars.

_Dimensions._--Length of the shell 0.2, height 0.17, breadth 0.1.

_Habitat._--Tropical Atlantic, Stations 341 to 349, surface, and at various depths.

{1723}7. _Conchidium magasella_, n. sp.

Shell laterally compressed, with two equal valves, very similar to the preceding species, but differing in the following characters:--Proportion of the three axes = 6 : 4 : 3. Margins of the valves with smaller and more numerous teeth; on each side of one valve sixteen to eighteen short conical teeth. Pores smaller and more numerous, about as broad as the bars.

_Dimensions._--Length of the shell 0.24, height 0.16, breadth 0.12.

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

8. _Conchidium productum_, n. sp.

Shell laterally compressed, prolonged, with two equal valves, similar to the two preceding species, differing in the following characters:--Proportion of the three axes = 3 : 2 : 1. Margins of the valves smooth in the anterior and posterior quarter, dentate in the middle lateral half; on each side of one valve ten to twelve strong conical teeth. Horns of the aboral hinge prolonged, conical, half as long as the shell, the ventral somewhat larger than the dorsal. Pores regular, circular, twice as broad as the bars.

_Dimensions._--Length of the shell 0.25, height 0.15, breadth 0.08.

_Habitat._--North Atlantic, Station 354, surface.

Genus 724. _Conchonia_,[340] n. gen.

_Definition._--#Concharida# with the lateral margins of the valves dentate, without sagittal keel, but with an apical horn on the poles of the sagittal axis, and with two caudal horns on the hinge (a dorsal and a ventral).

The genus _Conchonia_ is closely allied to the preceding _Conchidium_, its ancestral form, but differs from this and from all other Concharida in the development of horns on the poles of the sagittal axis. These are probably of great morphological importance, since they represent the beginnings of the hollow tubes arising from the poles of the sagittal axis in all Coelodendrida and Coelographida. In one of the three observed species each valve possessed an apical or sagittal horn, whilst in the two other species one valve only was provided with a horn. Since I observed one specimen only of each species, I cannot say whether this difference is important and of constant generic value.

1. _Conchonia diodon_, n. sp. (Pl. 124, figs. 10-12).

Shell laterally compressed, with two very unequal valves. Dorsal valve smaller, hat-shaped, on the apex with a fenestrated protuberance which is similar to the galea of the Coelodendrida, and bears a short, conical, backwardly-directed horn. Ventral valve larger, boat-shaped, without apical {1724}horn. Aboral hinge with two pyramidal, horizontal, caudal horns of different sizes, the dorsal smaller than the ventral. Lateral margin of each valve on one side with twelve to fourteen strong conical teeth (fig. 12). Lips of the narrow mouth thickened (fig. 11).

_Dimensions._--Length of the shell 0.3, height 0.27, breadth 0.21.

_Habitat._--Tropical Atlantic, Station 342, depth 1445 fathoms.

2. _Conchonia triodon_, n. sp. (Pl. 124, figs. 13, 14).