36 



In 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 is 

 capable of effecting it; and observation has taught me that 

 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. The 

 shell closes, not only at the side but top, the latter action 

 being effected by the separation of the pointed ends; and 

 the captive is either crushed to death or soon perishes from 

 confinement. It was formerly believed that the Pinnotheres 

 Pisum, a Parasitic Crab, had its residence within the shell 

 of the pinna, and was the friend to whose intimation the 

 pinna owed the knowledge of the presence of its prey. But 

 of the many pinnae I have had an opportunity of examining, 

 I have never found one containing this crab ; and Montagu 

 has made the same remark. In one instance three or four 

 pearls, perfectly round and of an intense black colour, were 

 found in the mantle near the hinge. 



* P. ROTUND ATA. Turt. Lin,, vol. 4, p. 302. This 

 species, not hitherto recognised as British, differs from 

 the last in form and texture : being more thin and brittle. 

 The free margin is less circular or cleaver shaped, the 

 sides being more parallel, and the open extremity simply 

 circular. In some specimens the lower surface is thickly 

 studded with concave spines, while others, even of less 

 size, are without them ; as is the case also, with the Pinna 

 ingens ; but no dependance, as specific marks, can be 

 placed on the presence or absence of these appendages. 

 The animal of this shell is sufficiently distinct from that 

 of the former, to confirm their specific difference. It is 

 less common than the former, and of somewhat less size, 

 but occupies the same situation. 



P. FRAGILIS. P. Pectinata. Mont. Test. Brit., vol. 1, 

 p. 178. P. F. Flem. Brit. An., p. 406. This is smaller, 

 and also much rarer, than either of the former. 



MALACEA. 



Ligament marginal, sublinear, either interrupted by inden- 

 tations or serial teeth, or wholly simple. Shell subunequal 

 valved, foliated. 



