GASTEROPODA. 



89 



presumed affinities. The huge univalve of the chalk, which 

 Sowerby called a Dolium, has been described as a Pteroeera 

 by d'Orbigny. In the tertiaries siphonated univalves abound, 

 and are mostly referable with certainty to recent genera. 

 The only marked change consists in the comparative abund- 

 ance of some scarce existing forms, and the absence or rarity 



Fig. 27. 

 Secondary Univalves. 



i. Spinigera, sp. ; Oxford Clay, Chippenham. 



2. Alaria Bentleyi, M. and L. ; Great Oolite, Collyweston. 



3. Purpurina Morrisii, Buv. ; Great Oolite, Minchinhampton . 



4. Neringea Bruntratana, Thurm. ; Corallian, Poland. 



5. Crossostoma Pratti, M. and L. ; Great Oolite, Minchinhampton. 



6. Trochotoma conuloides, Desl. ; Great Oolite, Minchinhampton. 



7. Neritoma bisinnata, Buv. ; Oxfordian, Ardennes. 



8. Pileolus plicatus, Sby. ; Great Oolite, Ancliff. 



9. Cinulia incrassata, J. Sby. ; U. Greensand, Blackdown. 



10. Acteonina concava, Desl.; Lias, Normandy. 



11. Bellerophina minuta, Sby.; Gault, Folkestone. 



of many now most conspicuous. Moreover, the geographical 

 distribution of the genera has undergone a great change since 

 the close of the eocene period. This change is most notice- 

 able in the cold-temperate zone, and is evidently the result 

 of altered climate. The northern seas must ever have been 

 inclement, and the tropical seas always tropical ; but the 

 latitude of England, being most liable to vicissitudes of 



