NATUKAL HISTORY SOCIETY. 77 



MOLLUSKS OF THE ATLANTIC COAST SOUTH TO 

 CAPE HATTERAS. 



I. Class Pelecypoda. (Bivalves, Lamellibranchiata.) Molliisks with a 

 bivalve shell. The two parts of the shell are somewhat similar in size, shape 

 .and material, and are usually joined together by a hinge. See Plate II. and 

 Key below. 



II. Class Gastropoda. (Univalves.) Mollusks having either a broad, 

 creeping disk called a foot on which it crawls, or two wing-like expansions from 

 the head by which it swims freely in the water. See Plate III. Figures 21-55 

 and Key on page 80. 



III. Class Cephalopoda. (Squids, Devilfish, etc.) Free swimming, oceanic 

 mollusks with a large, well separated head, having a circle of long, muscular 

 arms around the mouth. Eyes very large and perfect. Usually having no 

 external shell. See Plate IW. Figures 56-66 and Key on page 84. 



KEY TO THE FAMILIES OP MARINE PELECYPODS. 



* Shell when closed at the ventral side, gaping more or less at the ends, some 

 gap a little at one end, most gap at both ends. (N.) 



* Shell not at all gaping. (A.) 



A. With not over three cardinal teeth in each valve. (C.) 

 A. With many small cardinal teeth in each valve. (B.) 



B. With distinct radiating ribs ; large, over 30 long 5. ArcidcB 



B. Smooth, and the height and length about equal; under 12 long 



6. Nuculidce. 



B. Elongated shells, the length nearly or quite twice the height ; a pallial 



sinus 7. Ledidce, 



O. Valves equal in size, curvature and markings. (F.) 

 O. Valves unequal, or twisted so as to make them slightly so. (D.) 

 D. Two muscular impressions ; a distinct pallial sinus. (Q.) 

 D. One muscular impression; height about equal to or greater than the 



length. (E.) 

 ' E. Fixed mollusks with large, thick, rough shells, attached by the larger 

 valve to other shells, etc ?, 1. Ostreidce. 



