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PALAEONTOLOGY OF NEW-YORK. 
Ventral valve much the more convex, the greatest convexity being a 
little above 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- 
lateral 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- strige, 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, wher§ the muscular 
impression and rostral cavity are excessively developed; also in the 
larger and more symmetrical figures 5 and 6. 
