ZOOLOGY. 
ccxUii 
depression ; hinge of the right valve with one deep two-cut tooth and two lateral 
holes ; of the left, with two single teeth, and a central hole, for the reception 
of the tooth of the other valve ; lateral laminse, two in each valve, very small, 
only rudimentary ; margin of the valves crenated. 
In one specimen the teeth were reversed, for the right valve had two 
teeth, and the left but one uncut, but it differed in no other point as I could 
observe. 
Animal, mantle lobes separate, foot compressed flat, quadrangular. 
Tubes, none. 
Genus. — C rassina. Lamarck. 
Species 1 . — C. Semisidcata. Leach. Rosses Voyage. 
Species 2. — C. Arctica. 
Testa subrotundo-ovata, convexa, nigra, concentric^ striolata ; umbones 
subsulcatsB; lunula impressa oblongo-ovata; margine integerrimo. 
Crassina Arctica. Gray, ut supra. 
Shell roundish ovate, convex, white ; epidermis black, shining, finely 
concentrically striated ; umbones, rather prominent, very slightly concentri- 
cally furrowed; impression before the umbo, oblong ovate, deep, behind 
lanceolate ; hinge, right valve, with one strong single tooth, with a hollow on 
each side ; left, with two diverging teeth and a hollow in the centre ; 
lateral lamina, two in each valve, front strong near to the hinge, hinder 
remote, small ; edge even ; length one and half, depth one and quarter, 
breadth 3-4th of an inch. 
It differs from crassina semisnlcata, by being rounder, more convex, and 
not so much sulcated. 
F amily . — C ardi adaj. 
Genus. — C ardium. 
Shell obsokUly radiately ribbed, edge even. 
