60 LAND AND FRESH-WATER MOLLUSKS. 
inch in diameter, and is of a pale horn-colour, 
finely striated transversely, with three whorls. 
The animal of this handsomely-formed species 
(Pl. XI., fig. 143), like that of V. piscinalis, has 
a plume-like gill, furnished with about fifteen 
branches on each side, which is usually partially 
protruded on the right side when the animal is 
crawling; on the same side of the animal there 
is an accessory respiratory organ, in the form of 
a filament, arising from the mantle: in the pre- 
sent species this appendage is rather shorter 
than the tentacles, which it so much resembles ; 
its position, however, will not allow us to re- 
gard it as one. 
Famity NERITIDS. 
Gunus Neritina, diminutive of Nerita, a sea-snail. 
NeRITINA FLUVIATILIS— (the River Neritine) 
(Pl. IV., fig. 28)—is the only British repre- 
sentative of the large family Neritide of tro- 
pical seas and rivers, characterized by a thick 
semiglobular shell. The species of Neritina 
are more especially confined to rivers, and 
have small globular shells, coloured by bands 
or spots, and furnished with shelly opercula. 
The pretty speckled species found abundantly 
in many of the English rivers adhering to 
stones and to other shells, is about three- 
