the relative Age of Tertiai'y Deposits, 7 



with organic remains. I may however mention that there is 

 a large series of crag shells in the museum at York, from the 

 examination of which I believe his opinion has been formed. 

 In justice to M. Deshayes, I must now observe that there are 

 several individuals to whose judgement I should be disposed to 

 pay considerable deference, who think that in giving 40 per 

 cent, he has considerably underrated the proportion of recent 

 species, and that more than half, or perhaps three fourths of 

 the crag shells can undoubtedly be identified with species now 

 inhabiting the German Ocean. 



The Rev. Dr. Fleming, in a letter to Dr. Mitchell, F.G.S., of 

 London, in alluding to this subject, observes, *' Many of the crag 

 species are deep-water species, but I would fearlessly say they 

 are of British origin, and I make the remark, having been an 

 observer and collector of British shells for more than a quarter 

 of a century.'* 



In the annual address delivered by the President to the 

 Fellows of the Geological Society, Mr. Lyell particularly ad- 

 verts to the discordance of opinion between two such eminent 

 naturalists as Dr. Beck and M. Deshayes, and suggests that it 

 may probably be attributed to their difference of opinion as to 

 the amount of variation necessary to constitute a distinct spe- 

 cies. Thus, for instance. Dr. Beck would look upon those six 

 or eight forms which M. Deshayes includes under the name of 

 LiUcina divaricata as six or eight distinct speciesof the genus Lu- 

 cina, while M. Deshayes would consider them as varieties only. 

 Now this explanation is only admissible upon the assumption 

 that M. Deshayes allows the existence of as much difference be- 

 tween the craoj fossils and what he now regards as their living 

 analogues as there is between the six or eight varieties of the 

 Lucina divaricata. This is an important consideration; for if 

 M. Deshayes should assert the identification to be complete 

 between the crag fossils and living shells, it is evident that the ex- 

 planation offered affords no solution whatever of the difficulty. 



From these facts the following inference may, I think, be 

 fairly drawn : that if a series of tertiary fossils be placed before 

 the most eminent conchologists in different countries, for the 

 purpose of ascertaining from the pef-centage of extinct species 

 what position in a geological series the formation should hold 

 from which these fossils have been obtained, that position 

 might be decided to be, eocene in Denmark, Tw/ocd^w^ in England, 

 and pliocene in France ; and had we fifty intermediate grada- 

 tions it is very possible that no two conchologists would refer 

 the deposit in question to the same position. 



Greatly as the discordance of these results is to be lamented, 

 as retarding the progress of geology, it must mainly be attri- 

 buted to the present imperfect condition of conchological sci- 



A 



