Geology by Submarine Besearches. 325 



would be much more likely to be independent of catastrophes 

 and destroying influences, than such as had a more limited 

 distribution. In the cretaceous system, also, we find that 

 such species as lived through several epochs of that era, are 

 the few which are common to the cretaceous rocks of Europe, 

 Asia, and America. Count D'Archiac and M. De Vemeuil, 

 in their excellent remarks on the fauna of the Palaeozoic 

 rocks, appended to Mr Murchison and Professor Sedgwick's 

 valuable memoir on the Rhenish Provinces, have come to the 

 conclusion that the fossils common to the most distant loca- 

 lities, are such as have the greatest vertical range. My ob- 

 servations on the existing testacea and their fossil analogues, 

 lead to the same inference. It is very interesting thus to find 

 a general truth coming out, as it were, in the same shape, 

 from independent inquiries at the two ends of time. 



VII. Mollusca migrate in their larva state^ but cease to exist 

 at a certain period of their metamorphosis, if they do not meet 

 with favourable conditions for their development ; i. e. if they 

 do not reach the particular zone of depth in which they are 

 adapted to live as perfect animals. 



This proposition, which, as far as I am aware, is now put 

 forward for the first time, includes two or three assertions 

 which require explanation and proof, before I can expect the 

 whole to be received. First, that mollusca migrate. In the 

 fourth volume of the Annals of Natural History (1840), I gave 

 a zoo-geological account of a shell-bank in the Irish Sea, being 

 a brief summary of the results of seven years' observations at 

 a particular season of the year. In that paper, I made known 

 the appearance, after a time, of certain mollusca on the coasts 

 of the Isle of Man, which had not previously inhabited those 

 shores. They were species of limpet, about which there 

 could be no mistake, and one was a littoral species. At that 

 time, I could not account for their appearance. Many similar 

 facts have since come to my knowledge, and fishermen are fa- 

 miliar with what they call '* shifting" of shell-beds, which 

 they erroneously attribute to the moving away and swimming 

 ofi" of a whole body of shell-fish, such as mussels and oysters. 

 Even the Pec tens ^rtmch. less the testacea just named, have 



