202 LAND AND FRESH-WATER MOLLUSKS. 
groups: P. fontinalis to the typical section, 
characterized by the oval shell; the mantle is 
capable of being much extended, and its margins 
are frmged with long filaments; P. hypnorwm, be- 
longing to the subgeneric section Apleaa, has an 
elongated spire, and themargin 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. 
PHyYsA FONTINALIS—(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 
