Mrs. McKenny Hughes—Pleistocene Mollusca. 199 
ing beds of loam and sand in the same manner as at Barnwell Station, 
but not indiscriminately through coarse gravel and finer deposits as 
at Barnwell Abbey. 
The shells of Barnwell Station (Column III.) and Barrington 
(Column IV.) seem to represent marginal deposits such as are seen 
along the edges of ponds and rivers at the present time, occasionally 
encroached upon by river floods which swept them into holes and 
embayed corners. 
This is suggested by the accumulation of the smaller and lighter 
shells, just as now seen on the edge of the flood-water, and by the 
great abundance of Pupa marginata, while the rest of the shells 
are quite consistent with this view. Anodonta is rare and usually 
imperfect. ; 
The shells of Barnwell Abbey (Column I.) and of Grantchester 
(Column II.) seem to belong rather to the main channel of the river. 
Unio and Corbicula, which like river-beds, occur at Barnwell Abbey 
and Grantchester, but not at Barnwell Station or Barrington. Pupa, 
the characteristic shell of the other two localities, is comparatively 
rare at Barnwell Abbey and Grantchester. 
There is a marked agreement between the Barnwell Abbey and 
Grantchester Mollusca. All the species which are individually 
numerous in either locality are common to both. Those which are 
peculiar to one of the two localities are rare forms. Thus Hydrobia 
marginata, Mich., occurs at Barnwell Abbey, but has not been found 
at Grantchester. Limaz is recorded only from Barnwell. Single 
specimens of Planorbis nitidus, Miill., P. fortanus, Light., Patula 
ruderata, Miill., Helia lamellata, Jeff., Vertigo pusilla, Miull., V. 
edentula, Drap., V. minutissima, Hart., and Cecilianella acicula, Mill., 
have been found at Barnwell Abbey and there only. On the other 
hand, Planorbis nautilus, Linn., Helix obvoluta, Mill., and H. aculeata, 
Mill, are as yet peculiar to Grantchester. A more careful search 
would, however, probably result in filling up most of the gaps in 
both lists (see pp. 200—202) :— 
I must here acknowledge the kind help which I have received 
from Mr. Cooke in drawing up the list of shells, and in finding the 
range of various species. 
I am also much indebted to Mr. Dewick for the trouble he has 
taken in determining my specimens of Limaz and some other 
species, by comparison with those in the Natural History Museum, 
and also for lending me his rarer specimens for examination. 
Professor Rupert Jones has kindly determined the Ostracoda for 
me. He remarks that he has found Candona compressa, Koch, in 
Post-Tertiary beds in Berkshire, Cambridgeshire, also at Fisherton, 
near Salisbury, from the raised beach of Portland (Prestwich Coll.), 
and from the Chara Bed near Hitchin (Blackmore Coll.). Candona 
candida, Mill., he says, is very common, both Recent and Post- 
Tertiary. 
An explanation of the isolated or rare occurrence of certain forms 
in the ancient river deposits is suggested by what is seen at the 
present time along the Cam above Cambridge, where the artificial 
