Subfamily 3. Ommatodiscida, Stöhr, 1880, Palæontographica, vol. xxvi. p. 115

Definition.—Porodiscida without radial appendages of the concentrically annulated disk, but distinguished by one single or two opposite large marginal oscula, or wide openings on the margin of the disk, armed with a coronet of spines.

Genus 216. Ommatodiscus, Stöhr, 1880, Palæontographica, vol. xxvi. p. 115.

Definition.—Porodiscida without chambered arms and radial spines on the margin of the circular or elliptical disk, but with one large marginal osculum or opening surrounded by a coronet of spines.

The genus Ommatodiscus, together with the following Stomatodiscus, form the peculiar small subfamily of Ommatodiscida, founded by Stöhr in 1880 (loc. cit.). These remarkable Porodiscida, very nearly allied to Porodiscus, are distinguished by one or two large openings on the margin of the disk, and these "marginal oscula" are constantly armed with a coronet of spines (comparable to the osculum coronatum of Sycon in the Calcispongiæ). Probably in the living Ommatodiscida the osculum is the door from which a "sarcode-flagellum" issues (comp. above, p. ). Perhaps this osculum is comparable to the peculiar coronet of spines which is developed on one pole of the shell axis in some Ellipsida (Lithomespilus, Lithapium). The internal structure of the disk is commonly more or less obscure, as the lenticular disk is much thickened, sometimes nearly ellipsoidal. It is possible that the Ommatodiscida are more nearly related to the Lithelida than to the Porodiscida; but there is no indication of an internal trizonal medullary shell. Also the apparent resemblance to the Cyrtida is of no morphological value; both groups are of quite independent phylogenetic origin.

Subgenus 1. Ommatodiscinus, Haeckel.

Definition.—Disk circular.

1. Ommatodiscus decipiens, Stöhr.

Ommatodiscus decipiens, Stöhr, 1880, Palæontogr., vol. xxvi. p. 115, Taf. vi. figs. 8, 8a.

Disk circular, with two chambered rings around the spherical central chamber, of equal breadth. Chambers twice as high as broad. Pores very small, one-third as broad as the bars between them, two on the breadth of each ring. Osculum of the same breadth as the central chamber, surrounded by numerous very short teeth.

Dimensions.—Diameter of the disk 0.12; breadth of the central chamber and of each ring 0.03; pores 0.0015.

Habitat.—Fossil in Tertiary rocks of Sicily, Grotte, Stöhr.

2. Ommatodiscus stöhrii, n. sp.

Disk circular with three chambered rings around the spherical central chamber, of equal breadth. Chambers broader than high. Pores of the same breadth as the bars between them, four on the breadth of each ring. Osculum of the same breadth as the central chamber, surrounded by a corona of ten to twenty thin, bristle-shaped teeth, as long as its diameter.

Dimensions.—Diameter of the disk 0.2; breadth of the central chamber and of each ring 0.03; pores 0.004.

Habitat.—North Pacific, Station 244, depth 2900 fathoms.

3. Ommatodiscus circularis, n. sp.

Disk circular, with four circular chambered rings around the spherical central chamber, of increasing breadth; the fourth ring twice as broad as the second. Chambers about as broad as high. Pores large, twice as broad as the bars, one to two on the breadth of each ring. Osculum twice as broad as the central chamber, surrounded by a coronal of strong conical teeth, twice as long as its diameter.

Dimensions.—Diameter of the disk 0.18; breadth of the central chamber and inner rings 0.015, of the outer rings 0.03; pores 0.01.

Habitat.—South Pacific, Station 295, depth 1500 fathoms.

Subgenus 2. Ommatodisculus, Haeckel.

Definition.—Disk elliptical.

4. Ommatodiscus haeckelii, Stöhr.

Ommatodiscus haeckelii, Stöhr, 1880, Palæontogr., vol. xxvi. p. 115, Taf. vi. figs. 7, 7a.

Disk elliptical (6 : 7), with four chambered rings around the elliptical central chamber, of equal breadth. Chambers about as high as broad. Pores small, half as broad as the bars, two on the breadth of each ring. Osculum three times as broad as the central chamber, surrounded by a crown of strong conical teeth.

Dimensions.—Length of the disk 0.18, breadth 0.16; breadth of each ring and of the central chamber 0.02; pores 0.003.

Habitat.—Fossil in Tertiary rocks of Sicily, Grotte (Stöhr), Caltanisetta (Haeckel).

5. Ommatodiscus lævigatus, Stöhr.

Ommatodiscus lævigatus, Stöhr, 1880, Palæontogr., vol. xxvi. p. 116, Taf. vi. figs. 9, 9a.

Disk elliptical (3 : 4), with three chambered rings around the circular central chamber, the third ring half as broad as the second. Chambers twice as high as broad. Pores very small, one-third as broad as the bars. Osculum twice as broad as the central chamber, armed with a crown of short conical teeth.

Dimensions.—Length of the disk 0.15, breadth 0.11; breadth of inner rings 0.02, of the outer 0.01; pores 0.0017.

Habitat.—Fossil in Tertiary rocks of Sicily, Grotte, Stöhr.

6. Ommatodiscus fragilis, Stöhr.

Ommatodiscus fragilis, Stöhr, 1880, Palæontogr., vol. xxvi. p. 116, Taf. vi. figs. 10, 10a.

Disk elliptical (4 : 5), with five chambered rings around the elliptical central chamber, the fifth ring twice as broad as each of the others. Chambers about as high as broad. Pores very small, one-fifth as broad as the bars. Osculum three times as broad as the central chamber, surrounded by a coronet of short teeth.

Dimensions.—Length of the disk 0.17, breadth 0.13; breadth of the inner rings 0.01, of the outer 0.02; pores 0.001.

Habitat.—Fossil in Tertiary rocks of Sicily and Barbados; living in depths of the Tropical Atlantic and Pacific, Station 353, depth 2965 fathoms, Station 265, depth 2900 fathoms, &c.

Genus 217. Stomatodiscus, n. gen.

Definition.—Porodiscida without chambered arms and radial spines on the margin of the circular or elliptical disk, but with two large, opposite, marginal oscula, or openings surrounded by a coronet of spines.

The genus Stomatodiscus has the same characteristic structure as the foregoing Ommatodiscus, and differs from it only in the duplication of the large marginal openings. Whilst in the latter there is only one such marginal osculum, here we find on the margin of the disk two oscula, opposite on the poles of one equatorial axis.

1. Stomatodiscus amphistomus, n. sp.

Disk circular, with three concentric rings of equal breadth around the central chamber. Pores irregular, roundish, about two on the breadth of each ring. Surface of the lenticular shell spiny. On two opposite points of the margin a large osculum, three to four times as broad as the central chamber, armed with a coronet of strong pyramidal spines of different length, the longest equal to the radius of the disk.

Dimensions.—Diameter of the disk (with three rings) 0.12; breadth of each ring 0.016; pores 0.004.

Habitat.—South Pacific, Station 302, depth 1450 fathoms.

2. Stomatodiscus osculatus, n. sp. (Pl. , fig. 8).

Disk elliptical, nearly twice as long as broad, with three concentric rings around the elliptical central chamber, one piercing radial beam in the main axis, the other beams interrupted. Surface of the shell with small scattered thorns. Pores very irregular, roundish, partly aggregated in groups of four to eight smaller porules. On both poles of the main axis a large elliptical marginal osculum, about as large as the central chamber, armed with a coronet of short conical spines.

Dimensions.—Length of the disk (with three rings) 0.18, breadth 0.1; pores 0.001 to 0.006.

Habitat.—Western Tropical Pacific, Station 225, depth 4475 fathoms.