220 Mr. E. Forbes on a Shell-bank in the Irish Sea, 



The oysters are few, very large, muscular, and thick-shelled. 

 I have rarely seen a half-grown oyster on this bank, and never 

 a very young one. They seem to be the aged survivors of 

 some former colony. Many of the shells are dead. Those 

 of the living are generally perforated by that singular zoophyte 

 the Cliona, Oyster shells, when they die, do not generally 

 separate valve from valve : Venuses also remain in general 

 with the hinge perfect : but Tellinae and Psammobiae, as well 

 as Pectens, go to pieces. Cardium Icevigatum is often found 

 double, fresh-looking, of a large size, but always dead. The 

 one or two young specimens {Cardium serratum of British au- 

 thors) I have taken have been alive. Cardium elongatum and 

 nodosum occasionally occur, but also dead, and these two spe- 

 cies rarely remain double after death. Venus exoleta is rarely 

 taken alive here, but the specimens are fine, large, and double. 

 Young shells very rarely occur. Venus lineata is occasionally 

 found : on the opposite coast of the island it abounds. Venus 

 ovata is generally living. My a truncata is found only dead 

 and old ; the valves generally separate. I feel sure the My a 

 Swainsoni is only the young of this species, and not of Mya 

 arenaria as some have supposed. Wherever Mya truncata 

 is found, there Mya Swainsoni is found ; and all gradations of 

 form may be observed from Mya Swainsoni to Mya truncata^ 

 passing through the Mya ovata of Turton, which is the half- 

 grown shell. Look on the beak of a Mya truncata, and you 

 will find a little Mya Swainsoni included. The beak of bivalves 

 tells many tales against supposititious species, and is not yet 

 attended to as it ought to be. 



Occasionally taken, and on the whole not uncommon, are 



Fusus muricatus. Chiton Isevis. 



Trochus Montacuti. Pecten varius. 



Mactra elHptica. Solen ensis. 



Tellina crassa. Amphidesma tenue. 



Lucina undata and Radula. 



Mactra elliptica is generally dead, double and perforated : 

 Tellina crassa and Solen ensis always dead ; the former never 

 double. Pecten varius occurs in considerable plenty some 

 years, and at other times I do not find a single specimen. 



