so 



1 was assured, was taken near Falmouth, and am informed 

 that it is not uncommon at Helford, buried at some depth 

 in the sand. 



ARC ACE A. 



Cardinal teeth small, numerous, entering, and disposed on 

 each valve in a straight, arched or broken line. 



ARCA. 



GENERIC CHARACTER: Shell transverse, the valves 

 nearly equal, sides unequal, beaks distant, separated by 

 the facet of the ligament. Hinge linear, straight, without 

 ribs at its extremities, and furnished with a series of many 

 entering- teeth. Ligament wholly external. 

 * A. NOJ3. Tuft. Lin. Bnrlase's Nat. Hist. Corn., pi. 28, 

 fig. 16. A. Tortuosa. Pen. Brit. Zo., vol. 4, p. 97. 

 A. N. Mont. Test. Brit,, vol, J, p. 189, pi. 4, fig. 3. 

 Flem. Brit. An., p. 397. Scarce. I have obtained it 

 from Pridmoulb, nearFowey; and about the Land's end, 

 chief! v from cavities in stones from deep water. 

 A. LACTEA. Turt. Lin. A. Barbata. Pen. Brit. Zo., 

 vol. 4, pi. 58, fig. 59. But the reference misplaced to 

 A. Nucleus. A. L. Mont. Test. Brit., vol. 1, p. 138. 

 Flem. Brit. An., p. 398. 



" Tnis shell" says Montagu, "is found in great abundance 

 in some parts of the south coast of Devon ; and not fre- 

 quently on that of Cornwall; particularly about Falmouth; 

 but live shells, or double valves with the epidermis on, are 

 rare." It is scarce on other parts of the coast. 

 *A. FUSCA. Flem. Brit. An., p. 397; who quotes Borlase's 

 Nat. Hist. Corn., pi. 28, fig. 15; and Montagu's supple- 

 ment. Not uncommon in the crevices of stones from 

 deep water. 



The observance of a living specimen of this species 

 enables us to discern the variety of manner in which the 

 different genera of bivalves expand, or take their prey. The 

 area lies with the hinge downward, the hauriaut vessel 

 pointing obliquely up, with little powers of motion ; whereas 

 the C}cladae and Kelliae, walk and even climb, with celerily 

 and firmness, the hinge being upwaid, and the foot advanced 

 for the purpose of taking hold-last, and drawing it forward : 

 the heel at the same time being engaged in keeping it steady. 

 Pectens and Oysters preserve the hinge horizontal, the 

 former being capable of little, and the latter ot no motion. 

 When set at liberty, the first act of the Mytilus and Pinna, 

 is to secure themselves against future motion ; but the arose 

 have no better method of securing this state of rest, than 

 taking refuge in a cavity, from which their subsequent 

 growth ever after prevents their escape. 



