392 NATURAL SCIENCE. Dbc, 



nature that it has been unable to resist disintegrating influences, 

 and the other, that these genera were non-existent during that period. 

 The first hypothesis may account for the absence of these Pelecypoda, 

 owing to the fragihty and dehcacy of their tests ; the last, however, 

 cannot hold good in the case of Pleurotoma, seeing that its specific 

 development during the Tertiary period was so rich and varied that 

 it would be strange if it had not already been well established in 

 Cretaceous times. 



Closer examination into the distribution of the MoUusca reveals 

 facts of the highest scientific interest, not only tending to prove that 

 even in Cretaceous times their life-provinces were well marked and 

 defined, but also giving evidence of the individual migration of species 

 and of the gradual northern advance of whole groups. It seems 

 beyond doubt, from study of the Upper Cretaceous of southern 

 Germany, that the carnivorous Mollusca, such as Voliita and Conus, 

 genera so characteristic of the present tropical and southern seas, had 

 at the early part of that period obtained but a slight footing. But 

 in Upper Chalk times they form a striking feature, being rich in 

 species and widely distributed along the shores of the old Cretaceous 

 continent. It is a remarkable fact that, notwithstanding the varied 

 character of these occurrences in Germany, none of these genera had 

 been able to migrate either to Great Britain or France, the reason 

 evidently being that the great depression had proved an effectual 

 barrier to their advance, whether westward or northward. Only when 

 the elevation that closed the Cretaceous period had begun was this 

 barrier removed, and how suitable were the climatic conditions for 

 their extension at that time is evident from the great variety and 

 specific richness of these genera in the Tertiary strata. 



It may not unjustly be advanced that a discussion of this kind 

 should deal, not only with genera and species, but likewise with the 

 size and abundance of individuals. On these points, while fully 

 recognising their great importance, I regret that I have been unable 

 to obtain any really reliable and definite information as regards the 

 German fauna, though what I have gathered tends to confirm my 

 views ; indeed, the very wide distribution of so many species, both of 

 Gastropoda and Pelecypoda, especially in the more southern districts, 

 renders it in the highest degree probable that the molluscan fauna was 

 rich, not only in species, but also in individuals, though not com- 

 parable in this respect with the subsequent Tertiary fauna. 



A recent examination of the series of forms from the Blackdown 

 beds which is preserved in our national collection, has confirmed my 

 conclusions as to generic distribution ; for the Pelecypoda, especially 

 Trigonia, Cyprina, Cuatllcea, and Natica, are represented by specimens 

 of great size, while the Gastropoda, Tuvritella graimlata, Murex calcar, 

 and the beautiful little Aporrhais calcarata, are also in great abundance. 

 In the higher beds of the Cretaceous series both the Gastropoda and 

 Dimyaria rapidly diminish in size as well as number, the genus 



