34 
VALVATID^. 
swimming. Shell conical^ thin^ covered with 
an olive periostraca ; mouth rounds with a con- 
tinued simple peristome. Operculum horny^ 
suborbicular^ formed of many gradually enlarg- 
ing whorls, which have a raised membranaceous 
outer edge, forming a spiral ridge on the outer 
surface (f. 9. b.). 
The shells are known from Paludinse by the shelly 
cone being circular, and not bent in in any part by 
the proximate whorls. 
1. Valvata Muller. (Valve Shell.) 
Shell with the spire a little elevated, or flat and disk- 
like ; aperture quite circular, united all round, 
and furnished with a horny operculum marked 
with a single raised spiral membranaceous line. 
So named from the valve or lid which covers the 
orifice of the aperture. 
7. 1. Valvata piscinalis. Stream Valve-shell. — 
Shell globular, with an elevated obtuse spire, 
and a deep central umbilicus, (t. 10. f. 1 14.). 
The lingual plate of V. piscinalis is elongate; 
the central tooth sub- 
quadrate, base pro- 
duced on both sides, 
incurved, cutting edge 
triangular, the middle 
cusp with a pectinated 
lobe on each side ; late- 
ral teeth somewhat similar, laminaceous, toothed on 
Fig, 7. 
Teeth of Valvata. 
a, central ; b, c, d, lateral. 
