THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE III. VOL. 11. 



No. VI.— JUNE, 1885. 



OiaiC3-IISr.A.Ij JLIiTIGXilBS. 



I. — On the Land Mollusoa of the Eocenes. 



By J. Starkie Gardner, F.L.S., F.G.S., etc. 



(PLATE VI.) 



THE distribution of land moll u sea still remains one of the most 

 perplexing of the problems to be solved by the geologist. Sir 

 Charles Lyell seems to have been especially struck vi^ith their 

 capricious distribution, particularly in Madeira. The facts are still 

 substantially as stated in the tenth edition of his " Principles of 

 Geology," for Mr. Leacock, in show^ing me the results of many years 

 collecting in Madeira, observed that his researches had not modified 

 them in any important particulars. Nearly all the species are peculiar 

 to the Madeira Archipelago, and the remarkable fact about their dis- 

 tribution is that, though there are 5Q species in Madeira proper, and 

 42 in Porto Santo, only 12 are common to both islands, though in sight 

 of each other. Still more unaccountable it seems, that of 19 species 

 found on the Dezertas, three barren rocks which apj)ear but little 

 detached from the main island, only 12 are common to Madeira, and 

 even each of these islets has species and varieties peculiar to itself. 

 But two species of land-shells are in fact common to Madeira, the 

 Dezertas and Porto Santo. Sir Charles Lyell infers the great anti- 

 quity of the Archipelago from this, contrasting it with the far more 

 extensive group of the British Isles, numbering 200 islands, not one 

 of which have yet developed peculiar species. The fact that such 

 narrow seas have sufficed to keep the land-mollusca distinct conclu- 

 sively proves, in his opinion, that they have no ready means of 

 dispersal, and that their passage across even the narrowest sea must 

 be of such extraordinary rarity that the possibility need hardly be 

 taken into account. The presence of the same species throughout 

 the whole of the British Islands is thus the strongest argument in 

 favour of their having been joined together and to Europe at a very 

 recent date. 



Turning to that most fascinating book by Mr. Wallace, " Island 

 Life," in which all possible arguments ai'e arrayed to suj)port the 

 theory' of the Permanence of Continents, we cannot help remarking 

 how frequently the land-mollusca are opposed to other evidence, and 

 how, when the distribution of the rest of a fauna can be satisfactorily 

 accounted for, these crop up in contradiction. The present arrange- 



DECADE III. — VOL. II. — NO. VI. 16 



