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



to those which lie at low water on the shore, rolled masses of 

 porphyry, syenite, granite, slate, and limestone. The neigh- 

 bouring coast is sandy and marly, being in fact the edge, sea- 

 wards, of a great tract of Pleistocene strata, forming the 

 northern part of the island. The testaceous moUusca always 

 abundant on the bank and on its edge are the following : 



Pecten opercularis. Buccinum undatum. 



distortus. Trochus zizyphinus. 



Modiola vulgaris. tumidus. 



Hiatella rugosa, Nassa macula. 



Chiton cinereus. Emarginula fissura. 



Lottia pulchella. 



All the above are constantly taken alive in great numbers, and 

 dead specimens of them are also plentiful. The Buccinum un- 

 datum is generally of a large size, and almost always the va- 

 riety called Striatum by Pennant. Almost equally common 

 now with Emarginula fissura is Fissurella gr(Bca, but this was 

 not the case five years ago. They appeared suddenly on the 

 bank in considerable numbers and of a good size. I have 

 never taken but one young specimen (in which state it is Pa- 

 tella apertura of Montagu) though continually on the look out 

 for them. 



On the neighbouring shore I have observed a similar case 

 of a species appearing, which was not found there formerly. 

 Four years ago I picked up two specimens, very much sea- 

 worn, of Lottia testudinalis, on the shore at Ballaugh. At 

 that time no living specimens of the species were to be found 

 on our coast. Last year, however, to my surprise I found 

 great numbers of very young Lottia under stones at low 

 water. The year before I had taken a single full-grown ex- 

 ample at a low spring tide, the first Manx living specimen I 

 had ever found. This year (1839) the species is abundant; 

 three or four under every stone at low water mark, well-grown, 

 but not so large as the specimens common in Arran and the 

 Hebrides. What is the cause of this sudden appearance of a 

 species ? Are the tides, or storms, or the locomotive powers 

 of the molluscs themselves the means of transportation ? Mol- 

 lusca are much more active animals than is commonly sup- 

 posed ; and the smallest testacea, both bivalve and univalve. 



