Mr. S. V. Wood's Catalogue of Shells from the Crag. 243 



far more abundant than they are now ; Cyrena has disap- 

 peared, and Cyclas has dwindled into insignificance ; nor does 

 our weather hold out any prospect of bettering itself so as to 

 induce a return of the analogues of our ancient visitants. 



3rd. In a fossiliferous bed formed during a period when 

 the temperature of Britain did not exceed that of the warmer 

 regions of our world at present, there ought not to be the 

 same difference in the comparative number of species, extinct 

 and existing, in the marine and freshwater faunas, and scarcely 

 any in the case of the freshwater Pulmonifera. In such a 

 bed the freshwater Mollusks should either be nearly allied 

 to, or identical with, existing species of warmer climates. I 

 would refer to this rule the phaenomena of the shell-bed at 

 Grays, Essex, described by Mr. Morris, in which w r e find the 

 pectinibranchous Gasteropoda and the Acephala presenting 

 thermal characters, while the Pulmonifera are identical with 

 the existing British species. These phaenomena should lead 

 us to consider that bed as of pleiocene and not of pleistocene 

 origin. 



4th. When there is no positive but an evident negative 

 difference from the existing fauna in a tertiary or post-tertiary 

 freshwater deposit, our conclusions as to the climate of the 

 period in which it was formed must mainly depend on the 

 consideration whether the negation is of Pulmonifera or of 

 Pectinibranchia and Acephala ; for in the former case it pro- 

 bably depends on the action of secondary influences, and in 

 the latter it possibly may be owing to the same cause. 



5th. If in calculating percentages we deduce them from 

 lists including both freshwater and marine species, we draw 

 false inferences as regards the genera in the older rocks and 

 the species in the pleiocene and pleistocene beds. To correct 

 this error we should in the former case calculate separate 

 percentages for the marine and freshwater species, and in the 

 latter consider the freshw r ater Pulmonifera by themselves. 



XXVII.— A Catalogue of Shells from the Crag. By S. V. 

 Wood, Esq., F.G.S. 



To the Editors of the Annals and Magazine of Natural History. 

 Gentlemen, 



The following is part of a Catalogue of the fossil contents of 

 the Crag Formation, including the Conchifera of Lamarck. I 

 have endeavoured to make it as concise as possible, in order 

 (should you think it worth publication) not to trespass too 



r 2 



