ROOM XIII.] NATURAL HISTORY. 
87 
to marine bodies; being thus fixed, the foot is not fur¬ 
nished with a distinct disc for walking, but its end is ex¬ 
panded, flat, and orbicular, and as large as the mouth of 
the shell, which is generally protected by a horny opercu¬ 
lum; as the genera Vermelus and Spiroglyphus. The 
family of Cctpalidce have a simple conical shell. They are 
always attached to rocks by the back of the foot of the 
animal, which is folded on itself and unfit for walking 
upon. This back of the foot either secretes a shelly plate 
like an operculum, or forms a depression in the surface of 
the body to which it is attached, of the size of the shell, 
and marked with a crescent-shaped ridge, shewing the 
places where the muscle was fixed; as the genera Capulus 
and Hipponyxi Lastly, the family of the Crepidulce, 
(Crepidutidce ,) which have a very large and expanded 
mouth, and the inner lip very small and thin ; the latter 
is even sometimes entirely wanting; they have no oper¬ 
culum, and their gills form an oblique band across the 
front of the neck, as in the genera Crepidula, Calypircea, 
Cremona, and Dispotea : at the end of this family maybe 
placed, till the animals and their habits are better known, 
the genera Velutina and Phorus. The latter are peculiar 
for attaching to the outer surface of their shell, as it in¬ 
creases in size, stones, or fragments of other shells and 
corals; from whence they have been called respectively 
the Conchologist and the Mineralogist. The Phori have 
a subannular operculum, very like that of the Buccinum. 
Cases 42—48 contain the third order of Ctenobran- 
chous Gasteropodes, which are herbivorous, and have an 
entire mouth to their shells like the former, but, like 
the snail, they are hermaphrodite. The sides of the body 
are furnished with a series of filaments. Many of these 
have spiral shells, which are of a pearly lustre internally ; 
as the family of the Turbines, ( Turbinidce ,) which have a 
rounded mouth and a shelly operculum, as the genera 
Turbo, Imperator, and Pkasiaiiella. The family of the 
Trochi ( Trochidce) have a square mouth and horny spiral 
operculum ; as Troclius, Rotella, Monodonta and Solarium . 
The family of the Ear-shells ( Haliotidce ) are allied to the 
former, but they have a very expanded mouth, and no 
operculum ; most of these have a groove, a series of holes, 
or a canal in the outer lip over the gills, as in the genera 
