500 THE VOYAGE OF H.M.S. CHALLENGER. 



half as broad as the disk, without radial beams ; its pores of the same size as those of the disk, 

 regular, cu'cular ; 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) 012 ; breadth of the 

 first ring O'Ol, of the fourth 002 ; breadth of the girdle 0-05 ; pores 0-003. 



Habitat. — Fossil in the rocks of Barbados. 



Subfamily 3. Ommatodisoida, Stohr, 1880, Palasontographica, vol. xxvi. p. 115. 



Definition. — Porodiscida without radial appendages of the concentrically 

 annulated disk, l)ut distinguished by one single or two opposite large marginal oseula, or 

 wide openings on the margin of the disk, armed with a coronet of spines. 



Genus 216. Ommatodiscus,^ Stohr, 1880, Palseontographica, vol. xxvi. p. 115. 



Definition. — P o r o d i s c i d a 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 Stohr in 1880 {loc. cit.). These 

 remarkable Porodiscida, very nearly allied to Porodiscu.s, are distinguished by one or 

 two large openings on the margin of the disk, and these "marginal oseula" are constantly 

 armed with a coronet of spines (comparable to the osculum coronatum of Sycon in the 

 Calcispongise). 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 EUipsida (Lithomesiyilus, Lithapiuni). 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, Stohr. 



Ommatodiscus decipiens, Stohr, 1880, Palseontogr., vol. xxvi. p. 115, Taf. vi. tigs. 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 

 ■' Ommatodiscus = Y)i>(k with eyes; oi/.y.it, Sinxof. 



