TO GEOLOGY. 41 



FAMILY MACTRACEA. 



GENUS MACTRA. Linnaus. 



M. 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 striae, which are minute, are 

 apparently, on the fracture of the beaks which remain, 



