part 3] mollusca ftcom the Cambridge gravels. 239 



It would thus appear thai 1 &2 are the oldest forms, while 3 

 made its appearance at a much later date. Though we have 

 referred 1 & 3 to IT. hispida, in all probability the former has no 

 genetic relationship to the living form, the ancestral form being 

 No. 3. 



Helicodonta obvoluta (Miiller). 



In the English Pleistocene this species is only known from these 

 deposits, one example having been found at Grantchester and two 

 obtained by us at Barrington. In a living state in these islands it 

 is only known from Hampshire, West Sussex, and possibly Surrey. 



Ariaxta abbustobum (Linne). 



This species was very common at Barnwell Abbey, and, as usual, 

 it varied considerably in size; but the majority of the examples were 

 high-spired. One abnormal specimen in our collection measures 

 234 mm. in height by 20 mm. in breadth, and is a very heavy 

 shell, the thickening being very marked at the aperture. It 

 is, however, matched by a similar example from Upnall, Ilford, 

 although this last is a thin shell. This difference in texture 

 probably arises from the environment, for the soil on which the 

 Cambridge example lived was calcareous, while the Ilford individual 

 lived on a non-calcareous soil. 



Helix ntsmobalis Linne. 



The examples from Barnwell Abbey are well developed, and 

 usually the colour-bands are preserved. We have noted the fol- 

 lowing band formulae : — 



12345 (12345), 123 (45), 00345, 003 (45), and 00000. 



Vebtigo pabcedentata (Al. Braun). 



The occurrence of this species at the Downing Section is of 

 extreme interest. It is now known from Apethorpe, Northampton- 

 shire (late Pleistocene), Ponders End and Angel Road, Middlesex 

 (Glacial), and Elie, Fifeshire (Early Holocene). It has been 

 figured by us in this Journal. 1 



Clausilia pumila C. Pfeiffer (Ziegler IMS. ). 



This species was fairly common at Barnwell Abbey and Grant- 

 Chester, but rare at Barrington. It is not uncommon in the 

 remarkable Pleistocene deposit at Woodston, near Peterborough. 



SUCCINEA i' ni; is (Linne). 



Common at Barnwell Abbey, where it attained a large size. 

 Many examples may be referred to the var. limnoidea Pic. flare, 



and then only of small size, at Barrington, 



SrcciNKA elegans Risso. 



Common and well developed at Barnwell Abbey. 



1 Q. J. G. S. vol. xlviii (1912) pi. xvi, figs. G a & 66, 



