THE CONCHOLOGIST. 



THE GLACIAL PERIOD AND BRITISH NON- 

 MARINE MOLLUSCA. 



By H. E. QuiLTER. 



WHATEVER may liave been the cause 

 of the gradual chmatic refrigeration 

 which culminated in the last geological 

 period, called the Glacial Period, there is 

 abundant evidence that such an event 

 really did take place. The evidence would 

 appear to show that physiographical changes 

 were induced, bringing on a boreal con- 

 dition of the surrounding seas, and a 

 severely cold climate on land. It would 

 seem that it was the peculiar climatal con- 

 ditions of this Quaternary period that led 

 in the main to the present distribution of 

 the existing faunas and floras in Great 

 Britain. ^^'e have long been conversant 

 with the effect of this gradual alteration of 

 climate upon the vertebrate fauna, but not 

 until recently were we fully cognisant of 

 its effect upon the land and freshwater 

 mollusca, which inhabited this part of the 

 Continent, before Great Britain became an 

 island. 



The evidences show that during the 

 Glacial Period, where the English Channel 

 and the German Ocean are now present, 

 there were thickly wooded inland valleys, 

 irrigated by large freshwater rivers, among 

 which the present river Thames played an 

 important part. In the sands and gravels 

 deposited by this river at that time, are 

 found the remains of land and freshwater 

 mollusca then inhabiting the area which 

 the river drained. Mr. B. B. Woodward* 



* On the Pleistocene (Non-Marine) Mollusca of the London 

 District. By B. B Woodward, F.G.S., F.R.M.S., &c., Proc. 

 Geol. Assoc, Vol. xi., No. 8. 



Note on the Pleistocene Land and Freshwater Mollusca from the 

 Barnwell Gravels. By B. B. Woodward, F.G.S., Proc. Geol. 

 Assoc, Vol. X., No. 7. 



has fully worked out the molluscan fauna 

 from these beds in one district, and Mrs. 

 McKenny Hughest in another district. 



The results of their work show that, 

 while on the whole our present land and 

 freshwater mollusca were then existing, there 

 were a few species not now living in 

 Great Britain. These extinct species are : — 



Helix ruderata, Stud. This form has a 

 wide continental range at the present 

 time. 



Helix fruticum, Miill. Is found all over 

 Europe with the exception of Great 

 Britain. 



Clausilia pumila, Ziegl. Is found in 

 most parts of Ciermany, but more 

 common in the north. 



Hydrobia marginata, Mich. 



Unio littoralis, Lam. Found in nearly 

 all the rivers of P'rance. 



Corbicula fluminalis, Miill. This species 

 lives at the present time in the rivers 

 of Asia Minor and Syria, and the 



Nile. 



Pisidium astartoides, Sandb. 



\\\\\\ the exception of Unio littoralis and 

 Corhiciila fluminalis^ it is difficult to accoun 

 in a satisfactory manner for their extinc- 

 tion in the British area. The former is 

 apparently a more southern form, and the 

 Arctic climate of the Glacial Period would 

 not be favourable to its existence. C. 

 fluminalis is also a southern form, and 

 Mr. Woodward notes with regard to it, 

 that in the lower or older beds in which 

 it is found, the specimens are typical, 

 while in the newer beds they diminish 

 gradually in size, and finally disappear. 

 That the climatic conditions of the Glacial 



t On the Mollusca of the Pleistocene Gravels in the neighbour- 

 hood of Cambridge. By Mrs. McKenny Hughes, Geol. Mag. 



