Chapter XXXIX: Section IV: STICHOCYRTIDA, Haeckel, 1862, Monogr. d. Radiol., p. 280, 312 (14)

Shell laterally compressed, with two very unequal valves. Dorsal valve (fig. 14) larger, hat-shaped, on the apex with a large pyramidal horn which is half as long as the shell, curved and directed backwards. Ventral valve (fig. 13) smaller, boat-shaped, without apical horn. Aboral hinge with two pyramidal caudal horns of different sizes, the dorsal horn twice as long as the ventral. Lateral margin of each valve on one side with twelve to fifteen conical teeth. Perhaps the larger horned valve (fig. 14) may be the ventral, and the opposite smaller hornless (seen from above in fig. 13) the dorsal valve.

_Dimensions._--Length of the shell 0.21, height 0.17, breadth 0.12.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

3. _Conchonia tetrodon_, n. sp.

_Conchura tetrodon_, Haeckel, 1882, Manuscript.

Shell subspherical, with two nearly equal hemispherical valves, which are very thin-walled and similar in structure to those of the Coelodendrida, with very irregular roundish pores of different shapes and sizes (compare Pl. 121, fig. 3). Lateral margins of the valves with very numerous and irregular, thin, bristle-shaped teeth, similar to those of some Coelographida (compare Pl. 127, fig. 8). Aboral hinge with two equal, conical, caudal horns, which are straight, parallel, and half as long as the shell. Two similar straight conical horns are opposed on the poles of the sagittal axis, and arise from the apex of the two valves. This remarkable species may perhaps better represent a separate genus, _Conchura_, forming a direct transition to the ancestral form of the Coelodendrida, _Coelodoras_; it differs from the latter in the absence of a galea or hollow conical cupola on the apex of each valve, and in the solid, not hollow structure of the horns.

_Dimensions._--Diameter of the shell 0.24, length of the two sagittal horns 0.1, of the two caudal horns 0.12.

_Habitat._--Indian Ocean, Cocos Islands (Rabbe), surface.

Genus 725. _Conchopsis_,[341] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6.

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

{1725}The genus _Conchopsis_ and the following _Conchoceras_ differ from the other Concharida in the strong lateral compression of the shell, so that each valve is provided in the sagittal plane with a sharp prominent keel, comparable to the dorsal and the anal fin of fishes. These compressed shells are in general twice to three times as large as the more roundish and keelless shells of the five preceding genera. The sculpture of the fenestrated valves is extremely elegant. _Conchopsis_ possesses at the aboral hinge not the two prominent caudal horns, which mark the following genus _Conchoceras_, but in some species a peculiar ligament connects the aboral ends of both valves.

1. _Conchopsis orbicularis_, n. sp. (Pl. 125, fig. 3).

Shell subcircular, lenticular, strongly compressed on both sides, nearly as high as long, its sagittal perimeter nearly circular; frontal and cinctural perimeter spindle-shaped. Borders of the two boat-shaped valves smooth in 0.4 of the oral part, and in 0.1 of the aboral part of their length, strongly dentated in the remaining 0.5 middle part; about twenty-five slender, straight teeth on each side of one valve, size of the teeth increasing from the aboral towards the oral pole. In the half lateral perimeter of the shell (along the right and the left borders of each valve) sixty to sixty-five pores, in the half sagittal perimeter (along the keel of each valve) eighty to eighty-five pores, in the half equator sixty to sixty-five pores.

_Dimensions._--Length of the shell 0.53, height 0.55, breadth about 0.2.

_Habitat._--South Atlantic, west of Tristan da Cunha, Station 333, depth 2025 fathoms.

2. _Conchopsis compressa_, n. sp. (Pl. 125, figs. 7, 8).

Shell lenticular, strongly compressed on both sides; proportion of its longitudinal diameter to the sagittal and lateral = 10 : 9 : 3, its sagittal perimeter elliptical (fig. 7), cinctural and frontal perimeter spindle-shaped (fig. 8). Borders of the two boat-shaped valves smooth in 0.3 of the oral, and 0.3 in the aboral part, dentated in the remaining 0.4 middle part; about forty to forty-four very slender teeth of equal size on one lateral edge of each valve. In the half frontal perimeter of the shell (along the border of the valve) sixty-four to sixty-eight pores, in the half sagittal perimeter (along one valve-keel) seventy to eighty pores, in the half equator forty-four to forty-eight pores. Ventral and dorsal pores linear, three to four times as long as the circular, lateral pores.

_Dimensions._--Length of the shell 0.6 to 0.8, height 0.55 to 0.72, breadth 0.2 to 0.3.

_Habitat._--North Pacific, between 30° and 40° north latitude (between Japan and San Francisco), in depths from 2000 to 3000 fathoms frequent, Stations 241 to 252.

3. _Conchopsis carinata_, n. sp. (Pl. 123, fig. 8).

Shell subcircular, lenticular, in the central half slightly compressed, nearly spherical, in the peripheral half strongly compressed, with a broad, hyaline, smooth keel on the sagittal plane. Borders of the two valves smooth in the 0.2 of the oral, and 0.1 of the aboral part, strongly {1726}dentated in the remaining 0.7 middle part; about fifty slender teeth of equal size on one lateral edge of each valve. In the half lateral perimeter of the shell about forty-five to fifty pores, in the half sagittal perimeter sixty-five to seventy, in the half equator thirty-two to forty pores. The pores are arranged in parallel curved rows, which are separated by high denticulate crests. Aboral hinge with a strong ligament.

_Dimensions._--Length of the shell 0.6 to 0.7, height 0.55 to 0.65, breadth 0.35 to 0.45.

_Habitat._--South Atlantic (east of Patagonia), Station 318, depth 2040 fathoms.

4. _Conchopsis lenticula_, n. sp. (Pl. 123, fig. 9).

Shell subcircular, lenticular, strongly compressed on both sides, with a sharp and broad hyaline keel in the sagittal perimeter. Borders of the two valves smooth in 0.3 of the oral, and 0.2 of the aboral part, strongly dentated in the remaining 0.5 middle part; about thirty strong, conical teeth on the lateral edge of each valve. In the half lateral perimeter of the shell fifty to fifty-five pores, in the half sagittal seventy to seventy-five, in the half frontal forty to forty-five pores. The large central capsule of this species fills up the posterior half of the shell-cavity, the dark green phæodium the anterior half; the latter contains numerous peculiar, longish, nucleated cells (fig. 9_a_), parasites or symbiontes (?). Aboral hinge of the shell with a strong ligament.

_Dimensions._--Length of the shell 0.7, height 0.6, breadth 0.3.

_Habitat._--Central Pacific, Stations 271 to 274, depth 2350 to 2750 fathoms.

5. _Conchopsis pilidium_, n. sp. (Pl. 125, fig. 9).

Shell ovate, lenticular, compressed on both sides, with a broad, wing-like sagittal keel. Proportion of its longitudinal diameter to the sagittal and lateral = 6 : 5 : 3. Sagittal perimeter elliptical. Borders of the two hat-like valves smooth in 0.15 of the oral, and 0.2 of the aboral part, strongly dentated in the remaining 0.65 middle part; about twenty-five to thirty teeth of nearly equal size on one side of each valve. In the half lateral perimeter of the shell (along one border of each valve) forty-five to fifty pores, in the half sagittal perimeter (on the keel of one valve) seventy to seventy-five pores, in the half equator thirty-six to forty pores. Each pore is surrounded by a hexagonal frame. The opening of each valve in this species is bordered and partly closed by a broad, horizontal diaphragm or velum, like the deck of a boat; it is broadest on the oral side.

_Dimensions._--Length of the shell 0.78 to 0.8, height 0.66 to 0.7, breadth 0.3 to 0.4.

_Habitat._--South Atlantic, between Buenos Ayres and Tristan da Cunha, Stations 324 to 334, at depths between 1715 and 2900 fathoms.

6. _Conchopsis aspidium_, n. sp. (Pl. 125, figs. 1, 2).

Shell scutiform, strongly compressed on both sides, in the centre only lenticular, in the periphery wing-like, keeled. Proportion of the longitudinal diameter to the sagittal and frontal = 6 : 5 : 2. Sagittal circumference in the oral half semicircular, in the aboral half pentagonal, two {1727}acute corners jutting out near the aboral hinge, one corner in the keel of the dorsal, the outer in the keel of the ventral valve. Borders of the two boat-shaped valves smooth in 0.3 of the oral part, and in 0.1 of the aboral part, strongly dentated in the remaining 0.6 middle part; about thirty-five teeth in one side of each valve, larger on both ends than in the middle. In the half frontal perimeter of the shell sixty to sixty-five pores, in the half sagittal perimeter eighty to eighty-five, in the half equator of the shell forty to fifty pores.

_Dimensions._--Length of the shell 0.55 to 0.65, height 0.5 to 0.55, breadth 0.2 to 0.22.

_Habitat._--North Pacific, Stations 243 and 244, depth 2800 to 2900 fathoms.

7. _Conchopsis navicula_, n. sp. (Pl. 125, figs. 4-6).

Shell pear-shaped, compressed on both sides, in the sagittal periphery keeled. Proportion of the longitudinal diameter to the sagittal and lateral = 4 : 3 : 2. Its sagittal perimeter nearly ovate. Borders of the two boat-shaped valves smooth in 0.3 of the oral, and 0.15 of the aboral part, strongly dentated in the remaining 0.55 middle part; teeth conical, of nearly equal size. In the half frontal perimeter of the shell (along one border of each valve) forty to forty-five pores, in the half sagittal fifty-four to fifty-six pores, in the half equator thirty-two to thirty-six pores. Each pore is surrounded by a hexagonal frame, and pierces the shell in an oblique direction, dilated in the middle part (figs. 5, 6). Shell very thick-walled, several longitudinal crests on both sides of the keel of each valve. Hinge very strong, usually with a broad ligament between the two unequal aboral lips of the hinge.

_Dimensions._--Length of the shell 0.8, height 0.6, breadth 0.4.

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

Genus 726. _Conchoceras_,[342] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6.

_Definition._--#Concharida# with dentate lateral margins and a sharp sagittal keel of the compressed valves, and with two caudal horns on the hinge (a dorsal and a ventral).

The genus _Conchoceras_ has the same lenticular keeled and laterally compressed shell as the preceding ancestral genus _Conchopsis_, but is distinguished from it by the development of two large caudal horns on the aboral hinge. It bears therefore the same relation to the latter as _Conchidium_ does to _Conchellium_.

1. _Conchoceras caudatum_, n. sp. (Pl. 124, fig. 15).

Shell lenticular, slightly compressed; proportion of the longitudinal diameter to the sagittal and lateral = 6 : 5 : 4; sagittal and cinctural perimeter ovate, frontal perimeter elliptical. Free {1728}margins of the two boat-shaped valves dentate nearly in the whole perimeter; in one lateral border of each valve eleven or twelve very large conical teeth, the largest of which are nearly half as high as the valve. On the aboral hinge of the shell two divergent, very large horns, four-sided pyramidal, acute, straight, at the base perforated by a few large pores. The dorsal horn (of the smaller valve) is somewhat shorter than the ventral horn (of the larger valve). The apical distance of both horns is somewhat greater than their length, and about half the length of the shell. In the half frontal perimeter (along one border of each valve) thirty-two to thirty-four pores, in the half sagittal perimeter thirty-four to thirty-eight, in the half equator thirty to thirty-three. The pores are smaller near the girdle-fissure, irregularly quadrangular, and arranged in longitudinal rows, which are separated by meridional crests, and converge towards both poles of the main axis.

_Dimensions._--Length of the shell (without the horns) 0.24 to 0.26, height 0.20 to 0.22, breadth 0.16 to 0.18; length of the horns 0.1 to 0.13.

_Habitat._--Eastern part of the Tropical Atlantic, near the Equator, Station 348, depth 2450 fathoms.

2. _Conchoceras cornutum_, n. sp. (Pl. 124, fig. 16).

Shell lenticular, strongly compressed on both sides; proportion of the longitudinal axis to the sagittal and lateral = 4 : 3 : 2. Sagittal perimeter ovate, frontal perimeter elliptical, cinctural perimeter spindle-shaped. Free margins of the two keeled valves in the oral third smooth, in the remaining part strongly dentate; ten or eleven large teeth on one lateral border of each valve, the largest about half as high as the valve. On the aboral hinge of the shell two fenestrated apophyses which bear two stout, strongly curved horns, like pincers; the dorsal horn (of the smaller upper valve) is shorter and less curved than the ventral horn (of the larger lower valve). The lips of the mouth (at left on fig. 16) are also unequal, the upper lip emarginate. In the half frontal perimeter of the valve twenty-two to twenty-four pores, in the half sagittal perimeter twenty-eight to thirty, in the half equator twenty to twenty-two. The pores are separated by high parallel crests and arranged in longitudinal rows, which converge towards the aboral hinge.

_Dimensions._--Length of the shell (without the horns) 0.36 to 0.4, height 0.27 to 0.3, breadth 0.2 to 0.22; length of the horns 0.16 to 0.22.

_Habitat._--Tropical Atlantic, Station 338, depth 1990 fathoms.

Family LXXXIV. #COELODENDRIDA#, Haeckel (Pl. 121).

_Coelodendrida_, Haeckel, 1862, Monogr. d. Radiol., p. 360.

_Definition._--PHÆODARIA with a bivalved lattice-shell, composed of two hemispherical valves, a dorsal and a ventral. A conical cupola or a pyramidal galea arises from the apical pole of both valves, therefore at the opposite poles of the sagittal axis. Rhinocanna and frenula wanting. Three or more hollow radial tubes arise from each valve and are symmetrically disposed. Sometimes their branches form an outer {1729}bivalved mantle. The central capsule is so enclosed between the two inner valves, that its three openings lie in the open frontal fissure between them.

The family #Coelodendrida# differs from the preceding Concharida (its probable ancestral group) in the development of a conical galea or pyramidal cupola on the apical pole of each valve, and of three or more hollow radial tubes arising from each galea. They do not possess, however, the peculiar sagittal nasal tube or rhinocanna, which is constantly developed from the base of each cupola (and connected with its apex by a frenulum) in the following family, the Coelographida. These latter differ also from the former in the constant possession of prominent verticillate styles.

The family Coelodendrida was founded in 1862 in my Monograph (p. 360) and represented hitherto only by two species of the genus _Coelodendrum_, there described (p. 361, Taf. xiii. figs. 1-3, and Taf. xxxii. fig. 1). This first description, however, contained some errors, which were afterwards (in 1879) corrected by Richard Hertwig; this author also gave the first accurate description of the central capsule and its three openings. In the rich collection of the Challenger, the Coelodendrida are represented by four genera, but only seventeen species, some of which, however, are cosmopolitan and very common, particularly _Coelodendrum_.

The two valves of the lattice-shell, dorsal and ventral, are either hemispherical, or somewhat more flatly vaulted or cap-shaped. They are never connected in the equatorial zone of the body, as I supposed in my Monograph (1862, _loc. cit._); but they are separated by the girdle-fissure, a free circular equatorial interval, in which lie the three openings of the enclosed central capsule. Though the two valves, therefore, have no direct connection, they are, however, always opposed so accurately, that their equal free circular edges correspond exactly one to the other, so that the apex of each valve lies in one pole of the sagittal axis. From this apex there arises on each valve an irregular conical or three-sided pyramidal cupola, the galea (Pl. 121, figs. 3, 4, 8). The Coelodendrida differ in the possession of this galea from the Concharida, and agree with the Coelographida; but they never exhibit the peculiar rhinocanna or nasal tube, which arises from each galea in the latter family.

The siliceous lattice-plate of the two valves, and of the galea arising from them, is very thin and fragile, and its irregular roundish pores are extremely variable in size, number, and disposition. Sometimes the pores are so small and so scarce, that the plate appears nearly solid. At other times the siliceous plate seems to be really solid, and covered by a network of thin crests, the small dimples between which give to it the appearance of being fenestrated. Often the pores or the dimples are wanting in the central part of each valve, while they are very numerous and dense in the peripheral part. The same may be said of the lattice-plate of the galea, which is sometimes nearly solid, at other times richly fenestrated. The Coelodendrida agree in this structure with the following {1730}family, the Coelographida, and differ from the preceding family, the Concharida, in which the siliceous wall of the two valves is much thicker, and perforated by regular circular or roundish pores.

The galea or conical cupola in the apex of the two valves ("der kegelförmige Aufsatz" of the German authors) has in all Coelodendrida a triangular base and an irregularly conical or nearly three-sided pyramidal form. Its cavity is about one-third or one-fourth as large in diameter as the cavity of the hemispherical valve upon which it rests. The galea is relatively smaller and more irregularly formed than in the Coelographida, and differs essentially from that of the latter in the constant absence of a rhinocanna; there are also wanting, therefore, the characteristic frenula, which connect the nasal tube with the apex of the galea. The cavity of the galea probably always communicates with that of the valves by pores in the separating siliceous plate, and is besides pierced by irregular pores in its outer wall, very variable in form, size, and number, but it does not communicate with the cavity of the hollow radial tubes, from which it is separated by a thin, solid, siliceous plate.

The hollow radial tubes which arise from the galea in the Coelodendrida do not seem to possess that constant regularity in number, origin, and disposition, which is found in the following family, and there serves for distinction of genera. In my first description of the Coelodendrida (1862, _loc. cit._, p. 362), I pointed out this irregularity, and mentioned that the number of radial tubes arising from each galea varies from three to eight; the total number therefore amounts to from six to sixteen, the same minimum and maximum numbers which we shall encounter also in the radial styles of the following family. But whilst it is easy to determine the position and relation of these hollow tubes in the Coelographida, owing to the constant sagittal position of their rhinocanna, this task is very difficult in the Coelodendrida, where the rhinocanna is wanting. In the most frequent cases there arise from each galea three or four tubes, more rarely five or six, and very rarely seven or eight. The simplest and probably the original case is the development of three tubes, two of which are paired (divergent on the right and left), while the third is odd, lying in the sagittal plane. Perhaps these three primary tubes may be compared to the three cortinar feet of the NASSELLARIA, so that we may regard the two paired anterior as pectoral, and the odd posterior as a caudal tube. Usually the two paired or pectoral tubes arise from two corners of the triangular base of the galea, whilst the third odd or caudal tube does not arise from the third corner of the base, but more or less above it, and often even from the highest point or the apex of the galea. In the majority of species observed, this odd sagittal tube is forked even at its origin, so that two divergent tubes (an anterior and a posterior) arise from the apex of the galea (Pl. 121, figs. 3, 8). More rarely the two paired or pectoral tubes are also forked at the base, so that three pairs of tubes arise from each galea, and the total number of tubes amounts to twelve. Very rarely {1731}four separate tubes or four pairs of tubes arise from each galea, viz., two from the two anterior corners of the basal triangle, one from the posterior corner, and one from the apex of the galea. It is possible that this difference in the origin, furcation, and number of the hollow radial tubes may be employed for the distinction of genera of Coelodendrida, in the same manner as it is employed in the next following family, the Coelographida. But I have not been able, in spite of numerous and accurate examinations, to demonstrate in the former the same regularity in number and arrangement of the tubes as in the latter. It seems that these relations here are very variable, even in one and the same Species, and not yet fixed.

It is, however, probable, on the other hand, that the primary tubes (all or partly) are identical to the Coelodendrida and Coelographida. This is most probably the case with the posterior odd or caudal tube, which seems to be never wanting, and in both families is developed in the form of a dichotomous brush (never in the form of a verticillate style). Possibly also the two paired pectoral tubes are homologous in both families.

The hollow tubes are perfectly simple and unbranched only in one genus, _Coelodoras_, which is probably the common ancestral form of both families, and which may have been derived from _Concharium_ by development of a galea and tubes on the sagittal apex of the valves. All the other Coelodendrida have branched spines, and the ramification is constantly dichotomous, or repeatedly forked. There never occur in this family those characteristic "styles," or verticillate prolonged tubes, which we find in all Coelographida. Usually the cylindrical tubes are slightly curved and forked even near their base. The furcation is repeated a variable number of times in the different species. In the largest species each tube becomes a brush with more than one hundred terminal bristles.

We divide the Coelodendrida into two subfamilies, according to the different development of the distal branches of the hollow tubes. In the Coelodorida all the branches of the tubes remain free and are never connected by anastomoses, so that the surface of the bivalved skeleton is protected by the free radial distal branches of the tubes. In the larger species of _Coelodendrum_ (_e.g._, _Coelodendrum furcatissimum_, Pl. 121, fig. 1), the numerous branches of the dichotomous tubes form a dense thicket, similar to that in the Coelotholida.

In the second subfamily, Coelodrymida, the distal branches of the tubes are connected by numerous anastomoses, and compose either a simple lattice-plate on the surface of the skeleton (_Coelodrymus_), or a thicker envelope of spongy framework (_Coelodasea_). The lattice-mantle so produced is always bivalved, and its two outer hemispherical valves (dorsal and ventral) correspond exactly to the two inner valves, from which arise the hollow tubes. The free margins of the two external mantle-valves come externally into contact in the equatorial plane of the body, in which the girdle-fissure lies internally between the two central shell-valves. The free edges {1732}of the two external mantle-valves, opposed to one another in the circle of the equator, seem usually to catch one into another in the same way as the corresponding mantle-valves of the Coeloplegmida are loosely connected (Pl. 128, figs. 1, 7). A true concrescence between the two valves seems never to take place.

The two subfamilies of Coelodendrida therefore exactly correspond to the two subfamilies of the following family, the Coelographida. The Coelodorida and Coelotholida form in a similar way a thicket, by dichotomous ramification of the hollow tubes, all the branches of which remain free. The Coelodrymida and Coeloplegmida, on the other hand, form an outer lattice-mantle by anastomosing branches. The latter two subfamilies, of course, have been derived correspondingly from the two former, and the common ancestral form of all four is probably _Coelodoras_, derived from the Concharida.

Though the two corresponding subfamilies in both groups are very similar, they are, however, separated by important hereditary characters. All Coelodendrida (the Coelodorida without a mantle as well as the Coelodrymida with a mantle) possess no rhinocanna and no frenula on the galea, and they never develop prominent verticillate styles; the surface of their calymma is probably always spherical or subspherical. All Coelographida, however (the Coelotholida without a mantle as well as the Coeloplegmida with a mantle), possess a rhinocanna and frenula on the galea, and always develop prominent verticillate styles; the surface of their calymma is probably always symmetrically polyhedral.

The superficial armature of the skeleton in the Coelodendrida is rather simple, and by no means so manifold and differentiated as in the more highly developed Coelographida. The thin terminal branches of the hollow tubes are in the Coelodorida closed at the distal end, and armed with a variable number of short teeth (Pl. 121, fig. 2), or with a spinulate terminal knob, or a corona of recurved hooks (_ibid._, figs. 5-7). In the Coelodrymida, however, where the distal ends of the branches by anastomosing form the lattice-mantle, the spherical surface of this latter is armed with numerous thin spathillæ or radial bristles (often zig-zag or spinulate), and each bristle usually bears at the distal end a small anchor with two, three, or four recurved teeth; the outer convex edge of these teeth is usually smooth, the inner concave edge denticulate. All these ramules and branches of the tubes (also the thinnest terminal threads) are hollow, and filled up by jelly.

The _central capsule_ of the Coelodendrida does not lie outside the two central valves (as I supposed in my first description, in 1862, being deceived by the dark enveloping phæodium, Monogr. d. Radiol., Taf. xxxii. fig. 1), but it is enclosed between the two valves, as in the preceding and the following family. The first accurate description of it was given by Richard Hertwig in 1879 (_loc. cit._, p. 95, Taf. x. fig. 3). Its constant position between the two lattice-valves (dorsal and ventral) is such, that its three openings lie in the frontal plane, in the open fissure between the valves. The astropyle or the main-opening, with the radiate operculum and the tubular proboscis arising from it, lies on the anterior (or oral) {1733}pole of the main axis, whilst the two lateral accessory openings, or parapylæ, lie on both sides of the posterior (or aboral) pole, to the right and left. The position of the capsule is therefore the same as in the preceding Concharida (Pls. 123-125), and the following Coelographida (Pls. 126-128). The large nucleus, enclosed in the central capsule, is usually half as broad, and contains numerous nucleoli.

The calymma, or the extracapsular jelly-veil, is in the Coelodendrida usually spherical, very voluminous, and includes the entire skeleton, the thicket of the Coelodorida as well as the lattice-mantle of the Coelodrymida. Only the outermost terminal branches of the tubes in the former, and the radial bristles and spathillæ on the surface of the latter, remain free and project beyond the surface of the calymma. The phæodium is usually very large, three to four times as broad as the central capsule, and envelops it often completely. Usually it envelops only the anterior half of it, and the proboscis (Pl. 121, figs. 1, 9). Often numerous green, brown, or blackish phæodellæ are scattered through the whole calymma, and sometimes accumulate in a superficial layer on its surface. The galea of both valves is usually also filled up by the phæodium.

_Synopsis of the Genera of Coelodendrida._

-------------------------------------------------------------------------
I. Subfamily Coelodorida.
Hollow tubes, arising from the galea of both valves, simple or
dichotomously branched; the branches always free, not anastomosing.
No outer lattice-mantle.
-------------------------------------------------------------------------
Tubes simple, not branched, 727. _Coelodoras_.

Tubes forked or dichotomously branched, 728. _Coelodendrum_.
-------------------------------------------------------------------------
II. Subfamily Coelodrymida.
Hollow tubes, arising from the galea of both valves, dichotomously
branched; the branches anastomose and form an outer bivalved
lattice-mantle.
-------------------------------------------------------------------------
Lattice-mantle simple; its meshes lying
in a spherical surface, 729. _Coelodrymus_.

Lattice-mantle spongy; its meshes lying
in different planes, 730. _Coelodasea_.

Subfamily I. COELODORIDA, Haeckel.

_Definition._--#Coelodendrida# without an external bivalved lattice-mantle, with simple or branched hollow tubes, the terminal branches of which are free, not anastomosing.

Genus 727. _Coelodoras_,[343] n. gen.

_Definition._--#Coelodendrida# without external lattice-mantle, with simple, not branched, radial tubes, which arise separately from the galea.

{1734}The genus _Coelodoras_ is the simplest form of the Coelodendrida, and may be regarded as the common ancestral form of this and of the following family. It differs from all other members of these two families in the simple shape of the hollow radial tubes which arise from the galea, and are neither branched nor forked; the galea is very small, a flat triangular cap. _Coelodoras_ may be derived immediately from _Concharium_ or _Conchonia_ (p. 1723), by development of the galea and the radial tubes.

1. _Coelodoras hexagraphis_, n. sp.

Three straight, cylindrical, equidistant hollow tubes arise divergent from the three corners of each galea, and are about as long as the diameter of the valves, at the distal end armed with a spinulate knob. The odd sagittal (or caudal tube) is directed backwards, the two paired (or pectoral) tubes, forwards.

_Dimensions._--Diameter of the valves 0.16, length of the tubes 0.2.

_Habitat._--Central Pacific, Station 266, depth 2750 fathoms.

2. _Coelodoras octographis_, n. sp.

Four hollow cylindrical tubes, slightly curved, arise divergent from each galea, and are about one and a half times as long as the diameter of the valves, at the distal end knob-shaped, and armed with four crossed, recurved teeth. Two anterior (or pectoral) tubes arise from the two frontal corners of the galea basis, and diverge forwards to right and left. Two posterior tubes (a sagittal and a caudal) arise from the posterior corner of each galea, and diverge in the sagittal plane backwards.

_Dimensions._--Diameter of the valves 0.2, length of the tubes 0.3.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

Genus 728. _Coelodendrum_,[344] Haeckel, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 801.

_Definition._--#Coelodendrida# without external lattice-mantle, with branched radial tubes, the hollow branches of which are free and never connected by anastomoses.

The genus _Coelodendrum_ is the first described form not only of the family Coelodendrida, but of all #Phæoconchia# or bivalved PHÆODARIA; it is also the most common form of this group, and represented by ten different species, some of which are cosmopolitan, very common, and widely distributed. In my first description of _Coelodendrum_ I confounded it erroneously with some forms of _Coelodasea_ and _Coelographis_, the separated fragments of which I had found entangled between the branches of the former. The first figures of _Coelodendrum_ are given in my Monograph, in 1862, {1735}Taf. xiii. figs. 1-3 (not 4) and Taf. xxxii. fig. 1 (not 2 and 3). _Coelodendrum_ has been derived from _Coelodoras_ by furcation and repeated dichotomous ramification of the hollow radial tubes which arise from the galea.

Subgenus 1. _Coelodendridium_, Haeckel.

_Definition._--Ramification of the hollow tubes regularly dichotomous, each branch being forked again; therefore the two terminal ramules of the last branches equal.

1. _Coelodendrum ramosissimum_, Haeckel.

_Coelodendrum ramosissimum_, Haeckel, 1862, Monogr. d. Radiol., p. 363,
Taf. xiii. figs. 1-3.

_Coelodendrum ramosissimum_, R. Hertwig, 1879, Organism. d. Radiol., p.
93, Taf. x. figs. 3, 12.

Terminal branches regularly forked, with two equal, smooth, nearly straight or slightly curved fork-branches, diverging at right angles; their end-knobs with four crossed (or sometimes five or six) short recurved teeth.

_Dimensions._--Diameter of the spherical skeleton 1.2 to 1.8, of the two central valves 0.2 to 0.25.

_Habitat._--Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific; many Stations, surface and at various depths.

2. _Coelodendrum spinosissimum_, n. sp. (Pl. 121, fig. 7).

Terminal branches regularly forked, with two equal, smooth, straight, fork-branches, diverging at right angles, their end-knobs echinoidal, subspherical or club-shaped, with numerous short radial thorns.

_Dimensions._--Diameter of the skeleton 2 to 2.2, of the two central valves 0.25 to 0.03.

_Habitat._--Tropical Atlantic, Stations 346 to 349, surface.

3. _Coelodendrum furcatissimum_, n. sp. (Pl. 121, fig. 1-4).

Terminal branches regularly forked, with two equal, smooth, straight, or slightly curved fork-branches, diverging at acute angles; their end-knobs very small, with three short, diverging, conical teeth.

_Dimensions._--Diameter of the skeleton 2 to 2.5, of the two central valves 0.3 to 0.4.

_Habitat._--Cosmopolitan; Atlantic, Indian, Pacific; many Stations, surface, and at various depths.

4. _Coelodendrum bifurcum_, n. sp.

Terminal branches regularly forked, with two equal, smooth, more or less curved fork-branches, diverging at acute angles; their end-knobs thin, with two slender, parallel, bristle-shaped teeth.

_Dimensions._--Diameter of the skeleton 0.6 to 0.8, of the two central valves 0.1 to 0.15.

_Habitat._--North Pacific, Stations 252 to 256, surface.

{1736}5. _Coelodendrum gracillimum_, Haeckel.

_Coelodendrum gracillimum_, Haeckel, 1862, Monogr. d. Radiol., p. 364,
pl. xxxii. fig. 1.

Terminal branches regularly forked, with two equal, spinulate, curved fork-branches, diverging at obtuse angles and covered with numerous small thorns; their end-knobs cap-shaped, with a corona of six to eight small recurved teeth. In one specimen of this species (in 1859) I found entangled the fragments of _Coelographis gracillima_, figured in Taf. xxxii. figs. 2, 3, _loc. cit._ I supposed at that time, erroneously, that the latter belonged to full-grown specimens of the former.

_Dimensions._--Diameter of the skeleton 1 to 1.2, of the valves 0.15 to 0.2.

_Habitat._--Mediterranean (Messina), surface.

6. _Coelodendrum lappaceum_, n. sp.

Terminal branches regularly forked, with two equal, spinulate, straight or slightly curved fork-branches, diverging at acute angles and covered with small recurved hooks; their end-knobs large, conical, with a prominent apex and a basal corona of six to eight recurved teeth.

_Dimensions._--Diameter of the skeleton 1.5 to 1.8, of the valves 0.22.

_Habitat._--South Pacific, Stations 285 to 295, depth 1500 to 2600 fathoms.

Subgenus 2. _Coelodendronium_, Haeckel.

_Definition._--Ramification of the hollow tubes more or less irregular, mainly in the periphery; the terminal ramules of the last branches unequal.

7. _Coelodendrum cervicorne_, n. sp. (Pl. 121, fig. 8).

Terminal branches irregularly ramified, with unequal, smooth, curved ramules, diverging at obtuse angles, their end-knobs echinoidal, small, with short, diverging, conical thorns.

_Dimensions._--Diameter of the skeleton 1 to 1.2, of the valves 0.2.

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

8. _Coelodendrum digitatum_, n. sp.

Terminal branches irregularly ramified, with unequal, spinulate, straight ramules, diverging at acute angles; the two last fork-branches digitate, each with five diverging finger-shaped ramules, lying in a meridian plane; their end-knobs conical, pointed, with a corona of recurved hooks.

_Dimensions._--Diameter of the skeleton 1.6, of the valves 0.24.

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

{1737}9. _Coelodendrum flabellatum_, n. sp. (Pl. 121, fig. 6).

Terminal branches flabellate, irregularly ramified, each of the last two fork-branches being divided into four or five diverging straight branches of different lengths; usually the last eight or ten ramules lie in a meridional plane; their end-knobs cap-shaped, with a corona of recurved teeth.

_Dimensions._--Diameter of the skeleton 2.0 to 2.5, of the valves 0.25 to 0.3.

_Habitat._--North Pacific, Station 235 to 240, surface.

10. _Coelodendrum serratum_, n. sp. (Pl. 121, fig. 5).

Terminal branches flabellate, irregularly ramified like the preceding species; it differs from this in the strong compression of the broad, saw-shaped, terminal branches; the two opposite edges (placed in the meridional plane of the flabellum) are finely serrated; their end-knobs with a corona of diverging teeth.

_Dimensions._--Diameter of the skeleton 3.0 to 3.2, of the valves 0.3 to 0.36.

_Habitat._--Central Pacific, Stations 270 to 274, depth 2350 to 2925 fathoms.

Subfamily 2. COELODRYMIDA, Haeckel.

_Definition._--#Coelodendrida# with an external bivalved lattice-mantle, produced by the anastomosing branches of the hollow radial tubes.

Genus 729. _Coelodrymus_,[345] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6.

_Definition._--#Coelodendrida# with an external bivalved lattice-mantle, produced by the anastomosing terminal branches of the hollow tubes, which are connected in a spherical face.

The genus _Coelodrymus_, and the following closely allied genus _Coelodasea_, represent together the small subfamily Coelodrymida, differing from the Coelodorida in the possession of an outer bivalved lattice-mantle. They exhibit therefore the same relation to the latter, that in the following family the Coeloplegmida bear to the Coelotholida. The bivalved spherical mantle is composed of a simple lattice-plate in _Coelodrymus_, of a spongy framework in _Coelodasea_; the anastomosing branches of the hollow radial tubes become connected in the former in a spherical face, in the latter in the form of a spongy framework.

{1738}1. _Coelodrymus ancoratus_, n. sp. (Pl. 121, figs. 9, 10).

Network of the mantle loose, with large, irregular, polygonal meshes; the terminal branches of the forked trees, which communicate at the spherical surface of the calymma, and compose the mantle, are smooth. Spherical surface covered with very numerous and thin zigzag radial filaments, which are about as long as the galea, and bear at the distal end an anchor with two recurved teeth, denticulate at the concave proximal edge (fig. 10).

_Dimensions._--Diameter of the spherical lattice-mantle 2 to 2.5, of the valves 0.3 to 0.4.

_Habitat._--South-Eastern Pacific (off Juan Fernandez), Station 300, depth 1375 fathoms.

2. _Coelodrymus lappulatus_, n. sp.

Network of the mantle rather dense, with numerous and small irregular polygonal meshes; the terminal branches of the forked trees, which compose the mantle, are spinulate. Spherical surface densely studded with very numerous spinulate, radial filaments, which are about half as long as the galea, and bear at the distal end an anchor with four crossed recurved teeth, denticulate at the concave proximal edge.

_Dimensions._--Diameter of the spherical lattice-mantle 2.5 to 3, of the valves 0.4 to 0.5.

_Habitat._--South-Western Pacific (east of New Zealand), Station 169, depth 700 fathoms.

3. _Coelodrymus echinatus_, n. sp.

Network of the mantle very dense, with very numerous and small irregular roundish meshes; the terminal branches of the forked trees, which compose the mantle, are spiny. Spherical surface studded with very numerous, thin, radial bristles, which bear no anchor at the distal end.

_Dimensions._--Diameter of the spherical lattice-mantle 1.8, of the valves 0.22.

_Habitat._--South Pacific, Station 289, depth 2550 fathoms.

Genus 730. _Coelodasea_,[346] n. gen.

_Definition._--#Coelodendrida# with an external spongy lattice-mantle, produced by the anastomosing branches of the hollow tubes, which are connected in different heights.

The genus _Coelodasea_ differs from the preceding _Coelodrymus_ in the spongy structure of the outer bivalved mantle. The hollow branches of the radial tubes of _Coelodendrum_, which anastomose in _Coelodrymus_ only on the spherical surface of the calymma, and form a simple lattice-sphere, become connected in _Coelodasea_ in different planes (laterally and terminally), and therefore form an irregular spongy framework. The latter exhibits therefore to the former a relation similar to that which _Spongoplegma_ bears to _Carposphæra_ among the #Sphæroidea#.

{1739}1. _Coelodasea ramosissima_, Haeckel.

_Coelodendrum ramosissimum_ (_partim_), Haeckel, 1862, Monogr. d.
Radiol., p. 363, Taf. xiii. fig. 4.

Spongy framework of the spherical bivalved mantle very dense and thick, produced by very numerous, irregular anastomoses of the lateral and terminal branches, which arise from the hollow tubes. The last and thinnest terminal branches are forked, as seen in the radial section of fig. 4 (_loc. cit._), their ends are closed and armed with some very small denticles (not open, as figured in fig. 4). In my Monograph I had confounded this species with _Coelodendrum ramosissimum_, which however, may possibly be its ancestral form.

_Dimensions._--Diameter of the spongy spherical mantle 2 to 2.5, of the central valves 0.15.

_Habitat._--Mediterranean (Messina), surface.

2. _Coelodasea spongiosa_, n. sp.

Spongy framework of the bivalved mantle rather loose, not nearly so thick and dense as in the preceding species. The last and thinnest terminal branches are prolonged into denticulate, zigzagged, radial filaments, which bear at their distal end an anchor with two recurved teeth (similar to _Coelodrymus ancoratus_, Pl. 121, figs. 9, 10).

_Dimensions._--Diameter of the spongy spherical mantle 3 to 3.2, of the central valves 0.24.

_Habitat._--Equatorial Atlantic, Station 347, depth 2250 fathoms.

Family LXXXV. #COELOGRAPHIDA#, n. fam. (Pls. 122, 126-128).

_Definition._--PHÆODARIA with a bivalved lattice-shell, composed of two hemispherical valves, a dorsal and a ventral. A conical cupola or a helmet-shaped galea arises on the apical pole of each valve, therefore on the opposite poles of the sagittal axis. The cavity of the galea communicates with the sagittal rhinocanna, a peculiar nasal tube, which rests upon the valve, and is connected with the galea by a simple or double frenulum; its opening being directed towards the proboscis. Three or more branched hollow radial tubes arise from each valve, and are symmetrically disposed. Sometimes their branches form an outer bivalved mantle. The central capsule is so enclosed between the two inner valves, that its three openings lie in the open frontal fissure between them.

The family #Coelographida#, the last family of the PHÆODARIA, exhibits the highest degree of morphological development, not only in this group, but among all Radiolaria. They attain also the greatest size of all members of the class, since the diameter of their body is sometimes more than 20 mm., and in a few species even more than 30 mm. The complexity of their structure attains at the same time such a high degree, that they may be regarded as the most complicated, and (in a morphological sense) as the most highly developed of all Protozoa. Nevertheless their body always remains a single cell, {1740}and is closely allied to the preceding Coelodendrida; they differ from the latter mainly in the development of a peculiar new organ, the "rhinocanna," or "nasal tube." This is a hollow tube placed in the sagittal plane, arising from the base of each galea, and is connected with its apex by a simple or double frenulum. Between the oral openings of the two opposed rhinocannæ (one dorsal and one ventral) lies the proboscis of the central capsule.

The first observed species of Coelographida was _Coelographis gracillima_, some parts of which (but not the entire skeleton) were figured in my Monograph (1862, Taf. xxxii. figs. 2, 3). But I confounded these with _Coelodendrum gracillimum_, in the branched hollow trees of which the fragments of the former were entangled. I detected this error afterwards, when I had the opportunity of the observing some complete specimens. The first description of a complete skeleton was given in 1882 by O. Bütschli, who examined a large specimen of _Coelothamnus davidoffii_, captured by Davidoff in the Mediterranean (Zeitschr. f. wiss. Zool., Bd. xxxvi. p. 486, Taf. xxxi.). In the rich collection of the Challenger I was able to distinguish not less than nine genera and twenty-six species of Coelographida, but the majority of their large and most fragile skeletons were more or less injured, or quite broken. It was, therefore, of the highest importance for the minute study of this difficult group, that Dr. John Murray, during his expedition to the Færöe Channel (in 1882, in H.M.S. "Triton"), discovered in the Gulf Stream the beautiful _Coeloplegma murrayanum_, and brought up home hundreds of well-preserved specimens (Pl. 127). Only by the complete examination of this excellent material it was possible to answer many difficult questions as to their morphology, and to correct the errors in my description and in that of Bütschli.

We divide the family Coelographida into two rather different subfamilies, which may afterwards be separated as two divergent families, the Coelotholida (Pl. 122) and Coeloplegmida (Pls. 126-128). Both groups may be easily distinguished at a glance, since the numerous branches, arising from the hollow radial tubes, remain constantly free and independent in the former, and represent a spiny thicket, whilst in the latter they constantly become united, and by anastomosing form a peculiar "mantle," or outer envelope of delicate network. But besides, there are other and more important differences between the two groups. The peculiar hollow tube, arising from the base of the galea on each valve, which is filled with phæodella, and which we call the rhinocanna, develops in the Coelotholida on its open mouth two paired lateral frenula (right and left), which connect it (like two lateral bridges) with the base of two paired hollow main tubes (the "frontal tubes"). In the Coelospathida, however, the mouth of the rhinocanna develops a single odd frenulum only (in the sagittal plane), and is connected by it with the base of an odd, single, hollow main tube, directed forwards, the "nasal style."

The central bivalve lattice-shell, from which the galea and the tubes arise, exhibits in {1741}the Coelographida essentially the same form and structure as in the preceding Coelodendrida. The only (but important) difference between them is indicated by the constant presence of the peculiar rhinocanna in the former, whilst this is always absent in the latter. The two valves of the shell, dorsal and ventral, are either hemispherical or somewhat flatter, sometimes nearly cap-shaped, and formed of an extremely delicate and irregularly fenestrated plate of silica, as in the Coelodendrida. As in the latter, so also in the Coelographida both valves are of similar form and usually of equal size, but sometimes the dorsal is a little smaller than the ventral valve. The remarkable difference which Bütschli describes in his _Coelothamnus davidoffii_, and the inverse origin of the three tubes in both valves (_loc. cit._, Taf. xxxi; figs. 2, 4), depends upon an error of observation, produced by the artificial inversion of one valve, and the dislocation of their natural arrangement. The valves are never in direct contact, but separated by the equatorial fissure or girdle-cleft, in which the girdle zone of the enclosed central capsule and its three openings lie freely (Pl. 127, figs. 4, 5; Pl. 128, fig. 2). The free margins of both valves, which are opposite to one another, and bound the girdle-cleft, are always equidistant, so that the cleft in the whole equatorial circumference is of equal breadth. The margins are usually irregularly denticulate, sometimes armed with longer bristles (Pl. 127, fig. 8), more rarely smooth (fig. 5). The delicate lattice-work of the valves is always irregular and very variable, usually with numerous small and unequal pores, sometimes rudimentary, so that the valves appear partly solid and hyaline. The size of the valves is usually between 0.2 and 0.5 (in diameter).

The galea (_g_) or the apical cupola, which arises from the vaulted apex of each valve (or its sagittal pole) is more developed in the Coelographida than in the preceding Coelodendrida, and differs from the latter in the peculiar rhinocanna arising from its base, and in the single or double frenulum, connecting the open mouth of the rhinocanna with the odd or paired main tube arising from the galea. The two opposite galeæ lie therefore on the poles of the sagittal axis of the bivalve shell, and are so symmetrically disposed in the sagittal plane, that the open mouths of their rhinocannæ are directed towards the oral pole of the main axis, and nearly come in contact with the proboscis arising from the radiate operculum of the central capsule (Pl. 127, figs. 4, 5).

The size and form of the galea are very variable, even in one and the same species. The volume of its cavity is generally about as great as that of the hemispherical valve from which it arises, sometimes larger, at other times smaller. Its fundamental form is constantly dipleuric or bilateral, since the radial hollow tubes arise symmetrically on both its sides, and the rhinocanna proceeding from its base determines the sagittal plane. Usually the galea has the form of a vaulted helmet, the convex crest of which is inclined towards the mouth (Pl. 127, figs. 4, 5, 8, 9). Its anterior or apical part is broad and truncated in the Coelotholida, more or less conical in the Coelospathida; sometimes it assumes nearly the form of a bilateral three-sided pyramid, at other times it is more {1742}pear-shaped (Pls. 126-128). The thin siliceous wall of the galea has the same irregular and delicate network as the valve from which it arises, and sometimes the small irregular pores are also here reduced, so that the wall becomes partly solid. In some cases the thin, solid, siliceous plate of the galea and of the valve is covered by an irregular delicate network of crests; the dimples between these crests may be easily confounded with true pores.

The cavity of the galea is filled with phæodella and does not communicate with the cavity of the shell-valves, nor with the cavity of the radial tubes filled by jelly; it is closed towards the latter and the former by a thin solid plate of silex. Bütschli (1882, _loc. cit._, p. 488) describes in _Coelothamnus_ a large circular opening (Taf. xxxi. figs. 2_a_, 4_a_), and states that this is a direct communication between the cavities of the valves and of their galeas which are called by him "der dreiseitige kastenförmige Aufsatz" ([epsilon]). This error was caused by the fact that he observed the valves from the apical face only. The apparent opening of communication does not exist, and is the optical section of the rhinocanna, the shortened walls of which he describes as "trapezförmige Kiesellamelle" ([gamma]); the two lateral edges of the latter ("die seitlichen Zipfel," [delta]) are the paired frenula, which connect the open mouth of the rhinocanna with the base of the two frontal tubes.

The "rhinocanna or nasal tube" (Pl. 126, figs. 1, 4; Pl. 127, figs. 4-9_t_) is a very remarkable organ which is common to all Coelographida (without any exception), and distinguishes them markedly from all the other Radiolaria, and particularly from the closely allied Coelodendrida in which we find no trace of it. The rhinocanna is a cylindrical or three-sided prismatic hollow tube, which lies in the sagittal plane, on the outer surface of each valve, arises from the base of the galea, and is directed towards the proboscis of the central capsule. The two opposite rhinocannæ open on each side of the latter (Pl. 127, figs. 4-9_m_), and usually this "nasal mouth" or the anterior opening of the nose is somewhat dilated or even funnel-shaped. The posterior opening of the nose passes directly over into the base of the cavity of the galea.

Usually the rhinocanna is densely filled up by dark phæodella, which enter by this channel into the cavity of the galea (Pl. 127, figs. 4, 5, 9). Sometimes the entire phæodium is enclosed in the two galeæ and their rhinocannæ (figs. 5, 9), whilst at other times a great part of the phæodium lies outside of their cavities, and surrounds the proboscis of the mouth, or even the anterior half of the central capsule (fig. 4). The length of the cylindrical rhinocanna is usually about equal to that of the galea, whilst the diameter of the latter is from three to five times as great as that of the former. The structure of the thin wall is the same in both. The fine reticulation (fig. 8) is produced either by true, very small and irregular pores, or by a fine network placed on the solid thin wall. We may distinguish on each rhinocanna an outer or distal convex face, which is opposite to the proximal concave face of the galea, and an inner concave or proximal face, which rests immediately upon the convex outer face of the shell-valve; a thin solid lamella of {1743}silica here completely separates the cavities of the valve and of the rhinocanna resting upon it.

The "frenula or nasal suspensoria" (Pl. 127, figs. 4-9_b_) are thin ligaments of silica, which connect the nasal mouth (_m_) with the base of the main tubes arising from the galea; they are, therefore, also common to all Coelographida, and an exclusive and marked attribute of this family. They are, however, different in the two subfamilies of this group, corresponding to the different origin of the odd or paired main tubes. In the Coeloplegmida (Pls. 126-128) from the apex of each galea arises an odd main style, the nasal style (_g_ 1), and its base is connected with the nasal mouth by an odd frenulum (_b_). In the Coelotholida however (Pl. 122) the large nasal odd style is always wanting, and there arise two paired frontal tubes from the two corners of the truncate frontal face of the galea; therefore two paired frenula are developed (a right and a left), and these, converging towards the nasal mouth, connect its distal corner with the base of the two frontal tubes.

The odd frenulum of each valve of the Coeloplegmida lies therefore in the sagittal plane, whilst the two paired frenula of the Coelotholida lie on both sides of it, to the right and left. The frenula seem to be supporting columellæ or pillars, which support the fragile skeleton, and mainly effect a fixed prop for the fragile galea. In the Coelotholida the frenula are often rather broad and irregularly fenestrated lamellæ of silica (Pl. 122, fig. 2), whilst in the Coeloplegmida they are usually thin ligaments, fenestrated only at the broadened ends, which are inserted inside on the distal apex of the nasal mouth, and outside on the base of the nasal main styles.