310 
PALEONTOLOGY OF NEW-YOKK. 
there are appearances of faint undefined continuous striae. The speci¬ 
mens from the limestone are for the most part partially exfoliated, or 
have the shells silicified, by which the finer markings are obliterated. 
The interior of the ventral valve shows a perforation in the beak, 
opening below into an angular space which has been occupied by the 
beak of the dorsal valve, and thence communicating with the main cavity 
of the valve. The base of the fissure is margined on each side by a strong 
tooth which, extends in strong dental plates to the bottom of the cavity, 
and these are often continued in a thickened ridge bordering the muscu¬ 
lar impression. The imprints of the adductor muscles are opposite the 
bases of the dental plates, and below and on either side are the imprints 
of the broad divaricator muscle. 
In the dorsal valve the cardinal process is broad and strong, the crural 
bases somewhat widely diverging, and the centre abruptly depressed ; 
the teeth-sockets are large, and supported by strong lamellar callosities 
which extend along the inner side of the valve nearly parallel to the 
exterior margin. The muscular imprints are divided by a low distinct 
septum. 
This shell presents considerable variation in its forms and general aspect, though 
always preserving its unmistakable character. In young and half grown shells, 
where the hinge-line is but little extended, it has a general triangular form, as in 
figures 21 and 22 ; when the hinge-line becomes extended and the sides nearly 
straight with a moderately curving base, it is quadrangular, as in figures 18 and 19.. 
Figure 26, upon the ventral side, is somewhat hexagonal ; while figure 33 is 
decidedly pentahedral in outline. There is rarely some little elevation in the cen¬ 
tre of the mesial sinus, and sometimes a groove with an elevation on either side. 
The beak of the ventral valve is sometimes not closely pressed upon the dorsal 
umbo, but is usually so, and sometimes to such a degree that the apex is depressed 
into the groove of the mesial fold. 
I have heretofore ( Fifteenth Report on the State Cabinet, p. 147 ) called atten¬ 
tion to some peculiarities near the cardinal margin of this species. In a cardinal 
view of a well preserved specimen from the Upper Helderberg limestone of New- 
York (fig. 28), there is a ridge on the cardinal margin crossing the hinge-line with 
slight obliquity, and occupying a space on both sides. In a specimen from the 
Hamilton group, this fold is more oblique, rising from near the beak, as shoAvn 
in figure 35, Plate l ; while there is a second fold on the side of the shell. 
