88 



XII. On the Land and Fresh-water Mollusca of the 

 District around Colchester ; a contribution towards 



A LIST OF THE FaUNA OF EsSEX. 



By Henry Layer, M.E.C.S., F.L.S. 



[Eead September 24th, 1881. J 



The distribution of the Mollusca of a district depends so 

 miicli on its geological features that no excuse is needed in 

 prefacing an account of the Conchology of Colchester by 

 giving its geological conditions. The town itself, south of 

 the Eiver Colne, is situated on sands and gravels, and the 

 same soil is continued for five miles from that river in the 

 southerly district, and also for four miles to the west of the 

 town, as well as for two miles on the east. Beyond this 

 distance, in the latter direction, there are clays, sands, and 

 gravels of the Tertiary formations. Southwards the sands 

 and gravels are succeeded by London Clay, and westwards by 

 Boulder Clay and Brick Earths, as at Copford, w^here are the 

 well-known brick pits and the remarkable deposit of Shell 

 Marl. North of the town, after crossing the river valley, 

 London Clay appears again, and continues until the Stour is 

 approached, when sand is the prevalent formation. 



From this short sketch it will be seen that geologically the 

 district is one from which many moUuscan forms will be 

 wanting. There is an entire absence of bog, and on the 

 wdiole the district may be considered di-y ; but it is fairly 

 timbered, and contains a few woods, principally composed of 

 oak. The greater part is under the plough, and there are no 

 commons worth mentioning. With all these unfavourable 

 conditions, I think the following list will prove that we are 

 richer in species than might have been expected,^ 



1 [The number of species enumerated by Mr. Laver within his district 

 is 75, compared with 121 species given by Jeffreys in ' British Conchology ' 

 as the total number of Land and Fresh-water Molluscs at present in- 

 habiting Great Britain. — Ed.] 



