9§0 PALAEONTOLOGY OF NEW-YORK. 



Dorsal valve regularly convex, sometimes gibbous, becoming a little 

 depressed towards the base, the greatest convexity being a little above 

 the centre : there is usually a depressed line along the middle of the 

 valve. The hinge-line is about one-third, and sometimes half as long 

 as the width of the shell. 

 Surface usually smooth or very finely papillose ; but in its perfect con- 

 dition, it is covered by numerous fine seta which are matted together, 

 and the interstices being filled with clay, it has a rough appearance ; 

 while under a lens, these seta give a finely striate aspect. Beneath the 

 fine papillose surface, the texture of the shell is minutely punctate. 

 . The interior of the ventral valve presents beneath the beak a low 

 depressed area, or false area, which is bordered on each side by a strong 

 tooth. The muscular area is somewhat broad, flabelliform, with the 

 margins lobed ; the occlusor imprints in the centre are strongly marked 

 elongate oval spots, and there is a low median crest which often extends 

 to near the front of the shell. In the dorsal valve there is a strong wide 

 cardinal process, the inner face of which is mainly occupied by a broad 

 oval pad for the muscular attachment (see figure, Plate 45), extending in 

 a narrow callosity into the cavity below. 



The teeth sockets are deep, and margined by a strong callosity which 

 forms the base of the process ; and from thence proceed the crura and 

 the slender spiral arms, which make about eleven or twelve turns in the 

 larger individuals. The muscular area is narrow, well defined, and lobed 

 below ; while the inner portions, marked by the occlusor muscles, are 

 two sublinear spots. A slender crest extends along the middle of the 

 impression. The interior of the shell is somewhat pustulose. 



The figures given exhibit the variations in size and proportions usually observed 

 among large collections of specimens. 



Figure 33 is a cast from the Schoharie grit. 



Figures 34 and 35 are from the Comiferous limestone ; and the succeeding figures are of 

 Hamilton group specimens. 



The hinge-line of the two valves has been greatly enlarged, to show the parts descri- 

 bed ; and the casta of both valves have been enlarged two diameters, to show the character 

 and form of the muscular impressions. 



