WATER SNAILS. 2% 
the species are small: P. nautileus is the least, 
and P. corneus the 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. spworbis, 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 :— 
L Whorls all exposed... lit Ags, ee ON 
Last whorl embracing the rest, olossy staat rela» staat LAG 
9. ANCES (CUT) OR a a ea ee 
RPNeMACALINAUCE! fo ce. ces cee ac’ age we) 
3. memeber Tone. i.) ng sce cee ese LP  cOrmens 
eee Mera 6 he) oe i RL Lae Mo 
4, ) Whorls compact. ... ... LP. contortus 
* ( Last whorl proportionately large ... Pe et oe 
5 Shell covered with raised Tena stew P. albus 
* ) Shell smooth Be dassel Use) (awl epee eames SCO Oe 
6 Shell very depressed, of many narrow ae onallld i 
Shell few-whorled ; last whorl rather large... 8 
p 2 
