164 OYSTERS, AND ALL ABOUT THEM. 



occurs in the Clyde beds, as well as in the Red and Coral- 

 line Crag. Its extra-British range is considerable, extend- 

 ing from Norway to the Azores on the one side, and to 

 the ^gean on the other. 



In more northern seas this species soon fixes itself 

 permanently to various bodies by means of an agglutinating 

 secretion ; but in the Mediterranean and more southern 

 latitudes it usually remains free, or attached by a byssus 

 only, from which it has the power of withdrawing or dis- 

 engaging itself at pleasure. In the former or fixed state it 

 belongs to the genus Hinnites of Defrance. It has been 

 clearly shown, however, by Sowerby, on conchological 

 grounds, and by Dr. Fischer (in 1862), physiologically, 

 that this species is a true Pec fen, and that the genus Hin- 

 nites is not maintainable. The peculiar mode of attachment 

 by the shell in this case is the reverse of that adopted by 

 the oyster, the former having the smaller valve and the 

 latter the larger valve uppermost. 



The prickly scales are sometimes produced also on the 

 lower valve, and become leaf-like, as in the oyster. In 

 fixed specimens the byssal sinus is more or less closed ; 

 but I have some of a large size and much distorted, which 

 were attached by a strong byssus as well as adhering by the 

 shell. 



According to Fischer, the foot does not become atro- 

 phied or proportionally smaller in the fixed adult, although 

 it is then quite useless for the purpose of locomotion. The 

 fact is opposed to the general idea that the size of organs 

 is modified or affected by a change in the habits of an 

 animal. The present species was first described by Lister 

 with his wonted accuracy. Wallace, in his " History of 

 the Orkneys," has an ingenious way of accounting for the 

 shells being so distorted. He calls them the " twisted 



