146 LAND AND FRESH-WATER MOLLUSKS. 
tion of the Pleistocene fresh-water marls of 
Hssex, &c. With them is also Hydrobia mar- 
ginata, which, though not now found living in 
Britain, is repelled to Switzerland and Southern 
France. fH. hispida and H. arbustorum lived 
during the Mammiliferous Crag epoch; and 
H. pulchella and the extinct H. rysa as early as 
that of the Red Crag. H. pomatia is fossilized 
in the bone-caves of Belgium, and H. aspersa in 
those of Gibraltar ; and in the Uppermost Ter- 
tiaries at Newbury. The majority of the species 
occur in the sub-aérial deposits of the districts 
which they now inhabit. | 
Genus ButLimus. 
The animal of Bulimus presents no real dif- 
ferences from that of Helix; the body is elong- 
ated, the tentacles are rather shorter, and the 
dentition is Helicine: the transverse rows ex- 
tend in a straight line to about half their breadth, 
and then in a slight curve to the margin; the 
teeth are minute, and have a central, bluntly 
conical point, flanked on each side by a very 
much smaller but similar one. The shell is oblong, 
or turreted; the peristome is thin, or reflected. 
There are only three British species, belonging 
to two sections—JB. acutus, with a turreted shell 
and a simple peristome; B. montanus and DB. 
