204 PECTINIBRANCHIATA.—NERITID A. 
is curious, and has been particularly described by 
M. Bouchard Chantreaux. According to this careful 
observer, the Tentacled Bithinia lays from May to 
August. 
Famity NERITIDA. 
(Nerits.) 
A single British species represents a family 
which, in warmer climates, plays a conspicuous 
part along the shores of the ocean, and in their 
rivers and lakes. ‘The members have solid shells, 
more or less oval, the last whorl so greatly de- 
veloped as to occupy by far the principal portion, 
the aperture very open, somewhat crescent-shaped, 
with an expanded and flattened inner lip. 
The animals have broad muzzle-shaped heads, 
with awl-shaped tentacles, and eyes on short foot- 
stalks; the foot is somewhat three-sided, without 
any lateral filaments. An operculum is always 
found, which is spiral, semi-oval, and furnished 
with two internal processes on its front edge, form- 
ing a sort of hinge on the sharp edge of the inner 
lip of the shell. Dr. J. HE. Gray thinks that this 
peculiar structure of the operculum ‘“ makes this 
family more closely resemble the bivalve shells : 
the processes appearing to answer the same purpose 
(that of keeping the two parts in their proper situ- 
ation) as the teeth of the hinges in the bivalves.” 
GENUS NERITINA. 
The shell in this genus is semi-oval, without 
any perforation; the inner lip is slightly toothed, 
sharp-edged ; the surface is smooth, or striated, or 
spinous, covered with a skin. 
