PALAEONTOLOGY OF NEW-YORK. 



Dorsal valve generally the most ventricose, the greatest convexity a 

 little above the middle, curving abruptly to the sides, and often a little 

 depressed just within and below the cardinal angles, which are there 

 slightly inflected. Mesial fold very prominent, rounded, and marked 

 along the middle by a distinct rounded depression which is sometimes 

 very deep, though in rare examples it is represented by only a flat- 

 tening of the fold : beak rather prominent and well marked. Area 

 somewhat large, extending to the extremities of the hinge-line. 

 SuBFACE marked by about twenty or twenty-two simple low rounded 

 plications on each side of the mesial fold and sinus, often becoming 

 quite flattened near the front margin of old or full-grown individuals. 

 The entire surface of the plications, fold and sinus, is covered by 

 extremely fine interrupted longitudinal striae, which form numerous 

 minute pustules or short slender spines, giving a strongly granulose 

 aspect to the shell. The striae are seen only on extremely well pre- 

 served surfaces ; while the granulose character is a common feature, 

 except on exfoliated or worn specimens. Numerous concentric lines of 

 growth and finer concentric striae cross the plications, and often be- 

 come crowded near the front margin of adult specimens. 

 Interior of the ventral valve with a broad and somewhat quadrangular 

 muscular impression, which varies much in size in different individuals, 

 and is often marked in the upper part with strong transversely radia- 

 ting striae, and in the lower part with longitudinal striae. The upper 

 part of the impression is bounded by strong dental lamellae, which 

 unite with the broad teeth above. The upper part of the valve is 

 thickened, the shelly matter often filling the rostral cavity, and ex- 

 tending as a transverse septum in the bottom of the fissure for one-third 

 or one-half its length. In the interior of the dorsal valve, the muscular 

 impressions are often well defined, with a faintly marked mesial sep- 

 tum through the centre. 



This species is subject to considerable variation ; some specimens being but 

 moderatel}^ convex, and others extremely ventricose. In some the mesial fold is 

 scarcely flattened, while in others the central groove is nearly half the depth of the 

 fold. There is also much difference in the height of the area and in the incurving 

 fo the beak; while in some individuals the cardinal angles are extended consider- 

 ably beyond the sides cf the shell, and in others they are shorter and rounded. 



