202 LAND AND ERESH-WATER MOLLUSKS. 
groups: P. fontinalis to tlie typical section, 
characterized by the oval shell; the mantle is 
capable of being much extended, and its margins 
are fringed with long filaments; P. hypnorum , be¬ 
longing to the subgeneric section Aplexa, has an 
elongated spire, and the margin of the mantle plain. 
Two species of Physa , the most ancient of the 
genus, are known in the Purbeck formation; 
many occur in Tertiary strata, and the two 
living species are fossils of the newer Tertiaries. 
Physa eontinalis —(the Stream Bubble Shell) 
(PL XI., figs. 133, 141).—This pretty and inter¬ 
esting adjunct to an aquarium possesses an ex¬ 
tremely thin, glossy, and semi-transparent shell 
of a yellowish or brownish colour, of an oval 
form, and nearly half-an-inch long, and about 
half as much broad; the whorls are four, the 
first three extremely small, the last one occu¬ 
pies three-fourths or four-fifths of the shell. 
This species is widely distributed, and common 
on aquatic plants in brooks, ditches, canals, and 
slow-running rivers. 
It exhibits great activity, and its modes of 
progression are various. The animal glides 
rapidly along with a uniform quick motion, its 
narrow and elongated foot apparently inadequate 
to support the bulky body. The head, which is 
obtuse in front, supports two long tapering and 
very slender tentacles; the eyes are situated at 
