36 
la the latter case a rich harvest of shells is often afforded ; 
but the pointed end of the pinna is usually broken off by the 
violence. It is perhaps owing to the different degree of 
solidity in the ground, that the shells living in the deeper 
water, are so much less buried, than those of which Montagu 
speaks ; and one of the consequences may be a greater 
freedom of motion in the shell. Montagu observes, that 
the exposed end cannot be closed by art, but the animal 1 5 
capable of effecting it; and observation has taught me tha* 
this is its method of obtaining food. In its ordinary position 
this opening is about two inches wide, exposing the contained 
animal, which occupies but a small portion of the cavity, and 
seems to offer itself as a prey to the first creature that may 
choose to devour it. Some fish is thus tempted to enter> 
but the first touch within is a signal for its destruction. Tb e 
shell closes, not only at the side but top, the latter action 
being effected by the separation of the pointed ends; and 
tho captive is either crushed to death or soon perishes frou' 
confinement. It was formerly believed that tho Pinnothere® 
Pisum, a Parasitic Crab, had its residence within tho she! 
of the pinna, and was the friend to whose intimation 
pinna owed the knowledge of the presence of its prey. B 1 ' 1 
of the many pinnae I have had an opportunity of examining' 
I have never found one containing this crab ; and Montagu 
lias made the same remark. In one instance three or fo uf 
pearls, perfectly round and of an intense black colour, we>' 6 
found in the mantle near the hinge. 
* P. ROTUNDATA. Tin t. Lin., vol. 4, p. 302. Tl»' s 
species, not hitherto recognised as British, differs froi* 1 
the last in form and texture : being more thin and briUl®' 
The free margin is less circular or cleaver shaped, 
sides being more parallel, and the open extremity siwpB 
circular. In some specimens the lower surface is tliic^v 
studded with concave spines, while others, even of l eS 
size, are without them ; as is the case also, with the Piuj 1 
ingens ; but no dependancc, as specific marks, can u 
placed on llie presence or absence of these appendag 6 ®' 
The animal of this shell is sufficiently distinct from tl*? 
of the former, to confirm their specific difference. I* 1 
less common than the former, and of somewhat less si^ 6 ’ 
but occupies the same, situation, 
P. FRAGIL1S. P. Pectinata. Mont. Test. Brit., vol. 
p. 178. P. F. Flem. Brit. An., p. 40G. This is sni»l* e ’ 
and also much rarer, than either of the former. 
M A LACE A. 
Ligament marginal, sublinear, either interrupted by 1 
talions or serial teeth, or wholly simple. Shell subunefiU 9 
valved, foliated. 
