296 NERITID&. 
with a polished horny epidermis. They are mostly confined to 
the fresh waters of warm regions. One species (N. fluviatilis) 
is found in British rivers, and in the brackish water of the Baltic. 
Another extends its range into the brackish waters of the North 
American rivers; and the West Indian JN. viridis and meleagris 
are found in the sea. 
Some are amphibious, clinging to the roots of Nipah palms 
and other trees on the margins of rivers, while a few inhabit the 
foliage of tall trees that overhang the waters. 
NERITINA (restricted), Swainson. Shell globular or é6val-conic, 
usually brilliantly ornamented with colors; inner lip crenulated, 
rarely simple. Philippines, etc. 
THEODOXUS, Montf. (Vitta [Klein], Adams. Puperita, Gray. 
EHlea, Ziegler.) Shell transverse, smooth or nearly so; inner lip 
flattened, simple-edged or denticulated. Operculum, peg rudi- 
mentary. Inhabits the fresh waters of Europe. Kobelt has 
divided this group into NERITOGLOBUS, for species of the form of 
N. fluviatilis (xxviii, 66),and NERITICONUS, for the conical forms, 
like N. Mertoniana (1xxviii, 67). 
posT1A, Gray. (Mitrula, Menke.) Shell sandal-shaped, solid, 
the apex completely posterior and a little lateral; peristome con- 
tinuous and free; inner lip septiform, arcuated and denticulated 
in the centre of its margin. Brackish water, East Indies. J. 
crepidularia. Lam. (1xxviii, 68 ). 
CLYPEOLUM, Recl. (Neritella Humph.], Adams.) Shell glob- 
ular, oval or conie; thin, covered by a corneous epidermis; 
aperture semilunar; inner lip straight, flattened, smooth or den- 
ticulated on the margin; outer lip very full, often produced into 
a tongue upon the spire posteriorly. Operculum, peg and rib 
well developed, quite separated from each other. Mostly Poly- 
nesian. N. pulligera, Linn. (Ixxvili, 69, T0). 
cLitHon, Montfort. (Corona, Recluz.) Shell coronated with 
tubercles, or short or long spines, covered with a corneous epi- 
dermis; inner lip usually denticulated, presenting frequently a 
large superior tooth. Operculum with peg and rib both well 
developed, connected in half their length. The spines that 
usually ornament the whorls are tubular, and sometimes very 
long ; the Clithons inhabit tropical countries ; they crawl slowly, 
and only show during locomotion the tentacles and the tip of 
the muzzle; they prefer a stony bottom, clear and free from 
weeds, where the water is tolerably quiet. N. longispina 
(xxviii, 71). 
NERITONA, Martens. Peg of the operculum depressed, almost 
flat, lobate at its tip. WV. labiosa, Sowb. (Ixxviii, 72). Fresh water, 
Polynesia. Too close to Clypeolum. 
NERITODRYAS, Martens. Rib of the operculum deeply furrowed, 
multilobate at the tip, and deeply excavated beneath, Living 
