30 
I 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, enterin'!", 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 man/ 
entering teeth. Ligament wholly external. 
* A. NOjE. Turt. Lin. Borlase’s Nat. Hist. Corn., pi. 2 8, 
fig. 16. A. Tortuosa. Pen. Brit. Zo., vol. 4, p. 97- 
A. N. Mont. Test. Brit,, vol. I, p. 139, pi. 4, fig. 3. 
Flem. Brit. An., p. 397. Scarce. I have obtained it 
from Pridmoutb, nearFowey; and about the Land’s end> 
chiefly 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. 
Fleiu. Brit. An., p. 390. 
41 This shell” says Montagu, “is found in great abundance 
in some parts of the south eoast of Devon ; and not fre- 
quently on that of Cornwall; particularly about Falmouth; 
but live shells, or double valves with the epidermis on, arc 
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. 20, 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 specie 9 
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 hauriant vessel 
pointing obliquely up, with little powers of motion ; whereas 
the Cycladffl and Kelliae, walk and even climb, with celerity 
and firmness, the hinge being upward, 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, tb e 
former being capable of little, and the latter of no motion. 
When set at liberty, the first act of the Mylilus and Piut>a> 
is to secure themselves against future motion ; but the arc# 
have no better method of securing this state of rest, than 
taking refuge in a cavity, from which their subseq« eIit 
growth ever after prevents their escape. 
