TO GEOLOGY. 41 
FAMILY MACTRACEA. 
GENUS MACTRA. § Linneus. 
JM. dentata. Plate 1. Fig. 9. 
Description. Fragments only of this species have come 
into my possession. Two of them consist of a large portion 
of the hinge of the two valves. The one represented here is 
of the left valve, which is remarkable for a small lamellar 
tooth passing from the point of the beak directly between the 
cardinal tooth and the cavity of the ligament (fossette). 
The cardinal tooth is as usual angular, and the anterior 
portion is obliquely notched. Behind the beak, imme- 
diately under the dorsal margin, there exists, apparently, 
a long groove closed over near to the beak. The right 
valve is only perfect enough to exhibit the cardinal tooth 
and anterior portion of the hinge. The anterior portion of 
this tooth is divided into three distinct parts, all of which 
are lamellar. The groove on this anterior portion is en- 
tirely uncovered, and at the bottom is angular. I cannot 
with absolute certainty say that these two fractured hinges 
belong to the same species, but until perfect specimens are 
obtained they must be so considered. They both have a 
remarkably thin disk, in proportion to the largeness of the 
hinge; and the concentric strize, which are minute, are 
apparently, on the fracture of the beaks which remain, 
F 
