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 phenomena of the shell-bed at 
Grays, Essex, described by Mr. Morris, in which we find the 
pectinibranchous Gasteropoda and the Acephala presenting 
thermal characters, while the Pulmonifera are identical with 
the existing British species. 'These phenomena 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 freshwater Pulmonifera by themselves. 
XXVII.—A Catalogue of Shells from the Crag. By S. V. 
Woop, Esq., F.G.S. 
To the Editors of the Annals and Magazine of Natural History. 
GENTLEMEN, 
Tue 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 
