300 PALAEONTOLOGY OF NEW-YORK. 



Ventral valve much the more convex, the greatest convexity being a 

 little t^ovc the middle, becoming gibbous and extremely arcuate in 

 old individuals, curving abruptly to the sides and cardinal margins, 

 and more gradually to the front : umbo extremely prominent ; beak 

 neatly rounded and closely incurved, standing at a right angle with 

 the plane of the axis, or in old shells directed forwards. The anterior 

 portion is produced into a nasute or linguiform extension, usually 

 without a sinus or any depression of the surface. In the young or half 

 grown individuals, this feature does not appear. 

 Dorsal valve less convex than the opposite, moderately and regularly 

 convex in the young shell, becoming in the old shells gibbous above, 

 curving regularly to the sides, and often a little flattened at the baso- 

 latcral margins ; at about the middle of the length, or sometimes 

 above, the central portion of the valve becomes more gibbous, and 

 towards the front is abruptly elevated into a short rounded promi- 

 nent fold, corresponding to the linguiform extension of the opposite 

 valve. The beak is moderately incurved, lying close beneath that of 

 the opposite valve. 



The general aspect of the surface is that of a smooth shell with a few 

 concentric lamellose lines. In perfect specimens, however, the entire 

 surface is marked by fine close concentric stria;, and usually by indis- 

 tinct radiating striae, which are often more conspicuous in the partially 

 exfoliated shell, and still more distinct in some of the casts. 



The interior of the ventral valve preserves the generic characters in 

 a marked degree, in the open fissure beneath the beak, which is termi- 

 nated by a subcircular perforation ; in the strong dental plates, and 

 deeply marked muscular impression, as shown in fig. 18 of Plate XLViii. 

 The inner surface surrounding the muscular impression is radiatingly 

 striated. The reverse of these features is shown in the casts of smaller 

 individuals, from the Schoharie grit, figs. 3 and 4, where the muscular 

 impression and rostral cavity are excessively developed ; also in the 

 larger and more symmetrical figures 5 and G. 



