Zoology '65 



MOLLUSCA' 



The neighbourhood of Cambridge is very favourable for Mollusca. The 

 Fenland and the quiet waters of the Cam and its tributaries form a suitable 

 habitat for many freshwater species; thus, among the Gastropods, in addi- 

 tion to five species of Lymticva, no fewer than eleven members of the 

 Planorbidae have been found within about a mile of Cambridge. Theodoxus 

 Jluviatilis (Lin.) and various other operculate forms also live in the Cam 

 near the college bridges, while from Wicken as many as ten species of the 

 Pelecypod genus Pisidium have been recorded. Vertigo moulinsiana (Dupuy), 

 a scarce land snail restricted to marshy places, may also be found in Wicken 

 Fen, and Laciniaria biplicata (Mont.) lives close to the river not far from 

 Cambridge itself, although it is found in very few other places in the 

 British Isles. 



Upon the chalk hiUs to the south of Cambridge, on the other hand, 

 xerophilous species are common; Helicella virgata (da Costa), H. gigaxii 

 (Pfr.), and Monacha cantiana (Mont.) being especially abundant, the two 

 former showing much variation; while about 5 miles south of Cambridge 

 is found one of the very few British habitats of the large Helix pomatia 

 (Lin.). Helicigona lapicida (Risso) occurs at Fen Ditton, but it is rare in 

 Cambridgeshire, whereas its ally Arianta arbustorum (Lin.) is common and 

 has even been known to find its way into the roof of King's College 

 Chapel. 



Slugs are not exceptionally abundant in the neighbourhood of Cam- 

 bridge, but about eleven species have been found, and Agriolimax reti- 

 culatus (Miill.) and Arion hortensis (Fer.) are both very common, the former 

 varying greatly in colour. Moreover, all the eleven British species of the 

 Zonitidae, a family of snails related to some of the slugs, have been 

 recorded from the district. 



In gardens around Cambridge, Trichia striolata (Pfr.), Helix aspersa 

 Miill., and other forms are abundant, and in the University Botanic 

 Gardens six or seven exotic species have become estabhshed in the hot- 

 houses. Excluding these foreign introductions, about no species of land 

 and freshwater Mollusca are known to live in Cambridgeshire, as well as 

 two or three brackish-water forms that occur in the north of the County, 

 e.g. Hydrobia ventrosa (Mont.). 



Most of these no species are also present in the local Pleistocene and 

 Holocene gravels, but some of them have not been found in these deposits, 

 including certain species that are now among the commonest in the 

 neighbourhood, such as Trichia striolata (Pfr.) and Monacha cantiana 



' By Hugh Watson, M.A, 



