SS4 
PALEONTOLOGY 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. 
Surface 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 strise, 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 
moderately 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 of the shell, and in others they are shorter and rounded. - 
