58 
LAND AND ERESH-WATER MOLLUSKS, 
It can live in slightly brackish waters under 
tidal influence, as along the banks of the Thames 
below London. Dr. Gray says that the eggs are 
disposed on a tough strap-shaped green mem¬ 
brane, in a double row, consisting of six or seven 
pairs; the whole is fixed to the under side of 
aquatic plants. 
Genus Yalvata. 
Yalvata piscinalis —[the Stream Valve-shell ) 
(Pl.II.,fig.5),so named from inhabiting fishponds. 
The shell of this species is readily distinguished; 
it is globular, of four rounded well-defined whorls; 
colour brownish-yellow, very finely ridged, in a 
spiral direction; aperture circular, united all 
round, with a thin greyish-white operculum; 
there is a deep central umbilicus. Length one- 
fourth of an inch, and as much broad. The 
shell is very variable in the degree of elevation 
of the spire, and consequently in its diameter 
relatively to its height. The lingual ribbon of 
V. piscinalis is long; 
the central tooth (a 
fig. 8) is sub quadrate, 
with a produced base; 
hooked and denticu¬ 
lated ; the three un- 
cini (6, Cy b) are lan¬ 
ceolate, and toothed on each side. A common 
Fig. 8.—Teeth of V. piscinalis (Loven). 
