Page:Scientific results HMS Challenger vol 18 part 1.djvu/708

500 half as broad as the disk, without radial beams; its pores of the same size as those of the disk, regular, circular; three pores on the breadth of the first ring, six pores of the fourth ring.

Dimensions.—Diameter of the disk (with four rings, without the girdle) 0.12; breadth of the first ring 0.01, of the fourth 0.02; breadth of the girdle 0.05; pores 0.003.

Habitat.—Fossil in the rocks of Barbados.

Definition.— 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.

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. 407). 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.

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