94 Memoirs of the Indian Museum. [Vol. II, 



hairs. Maxilla very large, deeply excavated, the excavation being narrow and much 

 nearer the outer than the inner margin ; three stout spines external to the excava- 

 tion, internal to it five or six rather slender spines and a number of fine hairs; 

 internal margin broadly serrated, hairy. Mandible normally with five teeth, including 

 the inner angle, but this angle often dichotomous and the two teeth next it some- 

 times fused together, so that there might be either four or six teeth altogether; the 

 two mandibles of the same individual not always symmetrical, the outer tooth much 

 larger than and widely separated from the others ; the inner margin pectinate. 



This species, which was originally described from a single young individual, is 

 a very variable one. I have dissected about a dozen individuals and examined 

 over a hundred more externally. The three outlines in plate vii (figs. 5—7) illustrate 

 the manner of variation of the valves ; fig. 5 represents a very old individual and 

 fig. 6 a young one. 



A near relationship evidently exists between this species and Hoek's P. excavaUtm, 

 from which it is easily distinguished by the shape of the occludent segment of the 

 scutum. Both forms exhibit a marked similarity in their anatomy to the less degener- 

 ate species of Dichelaspis such as D. tridens. 



Subgenus Trii^asmis, Hinds (1844). 



On the whole I think it advisable to regard Hind's Trilasmis ehurnea as a type of 

 a subgenus. Its peculiarities are so marked that it might even be justifiable to regard 

 it as the type of a distinct genus. Hoek (1907) has, however, recently described a 

 new species {Pœcilasma obliquum) which is to some extent intermediate between 

 Trilasmis ehurnea and the more typical members of the genus Pœcilasma. In any 

 case Trilasmis^ in which Hoek's Pœcilasma obliquum should be included, may be 

 defined as follows : 



Species of Pœcilasma in which the tergum is absent or quite rudimentary and in 

 which the scutum is greatly developed and covers almost the whole of the capitulum. 



Pœcilasma (Trilasmis) eburneum, Hinds. 



Capitulum ovoid, broadly rounded above, the base pointed towards the occludent 

 margin ; external surface of the valves smooth. Tergum entirely absent. Scutum 

 occupying almost the entire surface of the capitulum, ovoid, pointed at the base, 

 inflated, not divided but having a distinct vertical suture not far from the occludent 

 margin. Carina short, arched, broad laterally, concave internally, terminating at 

 the base in a broad subquadrangular or circular disk directed downwards and inwards ; 

 a conspicuous ridge running down the valve on either side near its scutal margin and 

 becoming more pronounced in the region of the disk. 



Peduncle very short, placed obliquely as regards the capitulum. 



Cirri, etc. — First pair of cirri widely separated from second, with the rami slender, 

 subequal, each with about 10 joints, which are imperfectly developed at the base. 

 The other cirri rather short, curved, the bunches of hairs on the posterior margin of 



