APPENDIX. 339 



vastly larger in proportion to the size of the shell than they are in 0. satjittalis." I 

 cannot, however, find any hinge-area in the last-named shell, nor groove for the passage 

 of a pedicle ; this being visible only, so far as I can make out, in 0. ($)polita. 



Obolella sagittalis, Salter. PL L, figs. 1 — 14. 



Obolella sagittalis, Salter, M8. Report Brit. Assoc, for 1865, p. 285, 18G6. 



DisciNA lABiosA, Id. Ibid. 



Obolella sagittalis, Dav. Geol. Mag., vol. v, p. 309, pi. xv, figs. 17 — 24, 1868. 



^ec. CUar. Shell small, rarely exceeding two and a half lines in length and in 

 breadth ; almost circular, rather broader anteriorly ; front broadly romided ; beak in the 

 dorsal valve somewhat obtusely pointed ; posterior margin in the ventral valve nearly 

 straight, or slightly indented in the middle. Valves convex, and more or less deeply 

 marked by concentric lines of growth. In the interior of the dorsal valve two rather 

 large, irregularly circular, projecting scars, see fig. 5 a (a), are situated close to the 

 posterior margin, and are separated by a moderately elevated tongue-shaped ridge (c), 

 which extends to about two thirds of the length of the valve, and on either side, at about 

 half the length of the valve, are two smaller, oval, divaricating, slightly prominent scars 

 (d). (In the cast these projections form corresponding depressions; but they vary 

 considerably in their minor details, according to age and other conditions.) In the 

 interior of the ventral valve (fig. 10 a), two oval and oblique scars (a), smaller than the 

 corresponding ones in the opposite valve, and more widely separated, lie also close to the 

 posterior margin. A little lower down, two rather larger, but very slightly marked scars 

 (^) may be noticed; and between these four muscular impressions is a projecting A-shaped 

 ridge, with its most elevated portion (o) in the middle and between the first-named scars. 

 This leaves in the cast a deepish angular depression, which assumes, at first sight, 

 some resemblance to an apicial foramen. 



Ods. This well-marked species was named by Mr. Salter in 1865, but was not 

 figured or described. It is tolerably abundant in the condition of internal casts, which 

 are sometimes very sharply marked, so that I trust I have been able to define its internal 

 characters in a sufiiciently satisfactory manner. I was, moreover, able to demonstrate 

 that the so-called Biscina lahiosa (figs. 2, 13, 14) had been established on the internal 

 cast of the ventral valve of the species under description. This I ascertained beyond 

 doubt — first, from finding the casts of both valves of 0. sagittalis abundantly spread 

 over the same slabs ; secondly, from the casts agreeing exactly in their respective 

 dimensions ; and thirdly, from having, with the aid of gutta percha, taken moulds from 

 these casts attributed to I), lahiosa, it became evident that the holloAV, supposed to be due 



