414 BULLETIN OF THE 



former belong all the ancient paleozoic forms, which as far as known were all 

 Leptoidea. The majority of the living Chitons are also classed in this group. 

 The more specialized and peculiar, and especially the forms least embarrassed 

 by their shelly cuirass, or in which it has become more or less diminished in 

 size relatively to the whole animal, belong to the more modern group Opsi- 

 chito7iia. 



The Eochitonia will comprise four families, whose exact limits remain to be 

 defined by further researches, but which will for the present be regarded as 

 the equivalents of the lettered subdivisions of nearly the same name in my 

 paper on the Genera of Chitons (Proc. U. S. Nat. Mus., 1881, pp. 283-285). 

 They are the Leptoidea, Ischnoidea, Lophyroidea, and Acanthoidea. 



Family LEPTOCHITONID^. 



Genus LEPTOCHITON Gray. 



This group comprises the great majority of the deep-water Chitons. It 

 should not be confounded with Leptochiton H. & A. Adams, which is a hetero- 

 geneous assembly. Nearly all the species are white, with ferruginous or cine- 

 reous splashes, and of comparatively small size. They represent the paleozoic 

 Chitons, which were all Leptoids. Chiton eocenensis Conrad, from the Alabama 

 Eocene, is however a true Chiton of the restricted group typified by C. squa- 

 mosus Born, or C. tuherculatus Linne. 



Leptochiton pergranatus n. s. 



L. t. elongata. mediocre elevata, regulariter arcuata, jugo nullo; pallide cereo 

 tincta, interdum albida; valvis latioribus, apicibus nullis; v. ant. et post, plus 

 minusve concava; v. post, sine mucrone elevata; sculptura ut in L. cancellatus 

 sed granulis majoribus; areis lat. minus definitis; lam. suturalis, elongatis; 

 Z07m lata, squamuliis tenuibus, criniformes, dense obsita. Lon. 12.0; lat. 

 6.5 mm. 



Habitat, Station 192, near Dominica, in 138 fms., bottom temperature 

 63°.75 F. 



This fine species is nearest the Atlantic L. cancellatus Sowerby, and the 

 3 aipanese L.fidiginatus Ad. & Reeve. It differs from both in its concave or 

 excavated instead of convex terminal valves, in the absence or obsolete condi- 

 tion of the posterior mucro, in its much larger and more regular granules, and 

 in the subdepressed appearance also of the part of the median valves near the 

 girdle on each side. It is larger than cancellatus, and smaller than fuliginatus, 

 and without the dingy blackish painting of either. L. cancellatus is narrower, 

 higher, and with a sharper median angle. In L. fuliginatus the middle valves 

 are shorter from front to back, the sutural laminoe smaller and much more 

 triangular. There is no sign of a mucro on these valves, but in L. pergranatus 



I 



