NERITINA, TRACHELIPODA. 55 
very short ; with a subcentral erect vertex ; base concave, 
nearly orbicular, and somewhat cushion-shaped ; aper- 
ture semicircular, situate in the lower disk, and provided 
with a crenulated internal lip ; external lip furnished with 
a raised margin. 
Pileolus plicatus. Plate 1V. fig. 10 and 33. 
Only two species of this curious genus have been discovered ; they are both 
fossil, and were met with together at Ancliffe, in the coarse upper stratum of 
the Great Oolite, immediately under the Bradford Clay, and have since been 
found at Charter-House, Hinton, Somersetshire. 
Although these shells bear a strong resemblance to Patelle, their internal 
concealed spire connects them with the genus Nerita. 
Genus LX VIII. — NERITOPSIS, — Gray. 
Generic Character. — Shell subglobose, thick and can- 
cellated ; spire very small, consisting of three or four rapidly 
diminishing volutions ; aperture entire, transverse, oblique, 
suborbicular, or ovate ; outer lip thickened internally, some- 
what grooved and acute on its outer edge ; columellar lip 
thick, rather flat, and provided with a large broad double 
notch, a little rounded, above the centre of its inner edge. 
Neritopsis cancellata. Plate VI. fig. 18. 
The shells of this genus are distinguished from those of Nerita by that 
strongly marked character, the notch on the columella. 
The Neritopsi inhabit the seas of tropical climates. They have been found 
fossil in the Tertiary formations at Turin, Bordeaux, and several parts of 
Normandy. 
Genus LXIX. — NERITINA. — Lamarck. 
Generic Character. — Shell thin, external surface gene- 
rally smooth, and frequently covered with a strong horny 
epidermis ; spire mostly very short, sometimes nearly con- 
cealed, and at others obsolete ; aperture semicircular ; outer 
lip plain, sharp, and destitute of teeth, or crenulations, in- 
ternally, but within the lower region of the aperture it is 
provided with a somewhat elongated, transverse promi- 
nence, which seems the fulcrum for the articulation of the 
operculum ; inner lip flattened, and reflected on the colu- 
mella, and placed obliquely to the axis of the shell; edge 
generally sharp, and dentated or crenulated; as the animal 
increases in dimensions, part of the columellar lip is absorbed, 
which makes it appear as having no columella; operculum 
testaceous, semicircular, closing the aperture entirely, 
covered with a horny epidermis, and provided internally, at 
the lower end, with a tooth-like appendage, which fits into 
a hollow between the prominence and lip. 
