WATER SNAILS. 
211 
tlie species are small: P. nautileus is tlie least, 
and P. corneus tlie largest of the British species. 
Monstrosities of the species occur with an ele¬ 
vated spire. The prevailing colour of the shell 
is greyish or reddish. 
Coil shells are not known in a fossil state 
before the epochs of the Purbeck and Wealden 
deltas, two species of which occur in each of these 
formations. Species of Planorbis are among the 
most characteristic fossils of the Upper Eocenes of 
the Isle of Wight. Those at present living in this 
country are fossilized in the Upper Tertiaries of 
Grays, Clacton, &c., and at Clacton are associated 
with an extinct species, P. helicoides . P. margin - 
atus , P. spirorbis , and P. corneus are the most 
ancient of the existing species ; they occur in the 
Norwich Crag. 
The British species are characterized as fol¬ 
lows 
. (Whorls all exposed . ..... ... 2 
* ( Last whorl embracing the rest, glossy ... ... ... 10 
9 ( Whorls rounded .. ... .. 3 
’ ( Whorls carinated.. ... . 6 
^ j Diameter long ... ... P. corneus 
' l Shell small .. ... 4 
(Whorls compact ... ... .. ... P.contortus 
\ Last whorl proportionately large. .. 5 
g ( Shell covered with raised longitudinal striae P. albus 
* ( Shell smooth .. .... P.glaber 
„ | Shell very depressed, of many narrow whorls ... 7 
( Shell few-whorled; last whorl rather large ... 8 
p 2 
