NATURAL SCIENCES OF PHILADELPHIA. 27 



A careful examination of much better specimens of /. umbonatus, in the col- 

 lections now before us, than that first described by us, shows that form to be 

 even more closely allied to /. involutus of Sowerby than we had at first sup- 

 posed. As Sowerby's species holds a position, however, in the Upper Chalk, 

 and ours comes mingled with Lower Chalk types, it is probable a comparison 

 of perfect specimens of each would result in the discovery of constant differ- 

 ences. 



Locality and position. Same as last? 



Inoceramus tenuirostratus. 



Left valve very gibbous, subquadrilateral in outline ; anterior side truncated 

 almost immediately in front of the beak, and rounding into the base below ; 

 ventral margin semielliptical in outline ; posterior side rounded, or sometimes 

 subtruncate above ; hinge straight, comparatively long, and rather finely 

 crenulated, there being about five crenulations in the space of - 20 inch ; beak 

 very gibbous, pointed, slender, prominent, and distinctly incurved, directed 

 obliquely forward so as to bring its point nearly over the anterior margin ; 

 surface (of internal cast) smooth over the gibbous umbonal region, but having 

 a few small and very obscure concentric undulations below the middle. (Right 

 valve unknown, but probably compressed.) 



Length, 2*10 inches; height from base to hinge, 1 '82 inches ; height from 

 base to top of umbo, 2-13 inches ; convexity, (left valve only,) - 90 inch. 



This shell has a longer hinge and a more transverse form than any other 

 species known to us, having so prominent, slender, and incurved a beak. It 

 differs remarkably from the last, in the slenderness of its beak and less regu- 

 larly ventricose character of its left valve. Its umbonal region is perhaps 

 more gibbous than that of the last, though this gibbosity is more local and 

 abrupt. 



Locality and position. Same as last. 



Genus VENILIA, Morton. 



Venilia Mortoni. 



Shell transversely oblong, or subrhombic in outline, gibbous, thick and 

 strong ; base nearly straight, but rounding up in front ; dorsal margin parallel 

 to the base, excepting behind, where it rounds into the anal margin ; anterior 

 side truncated immediately in front of the beaks ; posterior margin truncated 

 a little obliquely ; postero-basal extremity abruptly rounded or subangular ; 

 beaks located directly over the anterior margin, directed obliquely forward, 

 and rather distinctly incurved at the points, as in Isocardia ; posterior um- 

 bonal slopes forming a prominent oblique ridge from each beak to the postero- 

 basal extremity ; lunule and escutcheon impressed, but without distinctly 

 defined margins ; surface marked with strong lines and more or less distinct 

 concentric ridges of growth, which latter assume a regular arrangement on the 

 umbones. 



Length, 1*66 inches; height, l - 40 inches; breadth or convexity, 1*17 inches. 



This species is allied to Venilia Conradi of Morton, (Synop. Org. Rem. pi. 8, 

 fig. 12,) but differs, if Dr. Morton's figures are correctly drawn, in being 

 proportionally longer transversely, more nearly oblong in form, and in having 

 its posterior margin more distinctly truncated, while its antero-ventral region 

 is less prominently rounded. Its dorsal margin is also more nearly horizontal, 

 and rounds less regularly into the truncated anal border. 



The genus Venilia was proposed by Dr. Morton, in 1834, for the reception 

 of a rather peculiar shell, which D'Orbigny afterwards referred to Cyprina. 

 We agree, however, with those authors who regard it as clearly distinct from 

 the typical forms of Cyprina, though it may not be generically distinct from a 

 few forms referred by some to that group. We have not had an opportunity 



1862.] 



