94 
PALAEONTOLOGY OF NEW-YORK. 
sloping to the margin, and flat or concave upon the umbo : hinge 
extremities often slightly deflected to the dorsal side. 
Area of the ventral valve from one to two lines in width, and variously 
inclined to the plane of the lateral margins of the shell, transversely 
striated and the inner margin crenulated for about half the distance 
from the centre to the cardinal extremity; the remaining portion 
marked by a narrow groove (the latter feature not always distinct). 
The foramen is closed, a smooth triangular space marking its place, 
with a narrow callosity along its centre. Area of the dorsal valve 
narrow and linear throughout. 
Surface marked by angular subequal interrupted or rugose striae, which 
are often made to appear more equal and uniform by the partial exfo¬ 
liation of the shell. The striae are bifurcated two or three times before 
reaching the margin, and they are sometimes increased by intercala¬ 
tion. As usually preserved, they rise at intervals into sharp ridges 
or elongate nodes; and in well-preserved specimens, these are pro¬ 
duced into short spines.* 
When the shell is exfoliated, the surface between the striae is marked 
by minute pores or tubular openings, which communicate with pustulose 
elevations on the interior surface of the shell. In the entire cast, these 
pores mark the surface with considerable regularity, varying in size, and 
uniformly larger below the middle where the valve is more abruptly 
deflected. 
The muscular impressions of the ventral valve are very large and broad, 
and the margins distinctly limited by a curving elevated border. It is 
divided in the middle by a more or less developed septum, which termi¬ 
nates in a prominent callosity beneath the place of the foramen : this 
callosity gives a bilobate character to the cast of the rostral cavity. The 
place of the occlusor muscles is strongly marked, and that of the divari- 
cator muscles distinctly lobed. The muscular imprints are about two- 
thirds as long as wide, and the width is often nearly equal to half the 
• This feature is rarely preserved in parts of the surface in our specimens; but Mr. Billings has figured 
a specimen with the entire surface covered with short spines (Canadian Journal, loc. cit.). 
