78 CLASS GASTEROPODA. 
whorl, and on the columella, the centre of which, besides, is 
hollow throughout. Nine species are already ascertained. 
M. Brongniart separates from the cerithia, the 
PoTAMIDA, Brong., 
Which, with the same form of shell, has a very short and 
scarcely emarginated canal, no furrow on the upper part of the 
right margin, and the external lip dilated. ‘The potamida 
inhabit rivers, or at least their mantles, and fossil specimens 
are found in strata which contain other fresh-water or land 
species only.—See Brongn, Ann. du Mus. xv. 367. In this 
subgenus should be placed the Cerithiwm atrum, Brug., and 
some other cerithia of the same writer, and among the fossils 
the Potamida Lamarckii, Brongn. 
MuREX, Lin., 
Comprises all those shells in which there is a salient and 
straight canal. The animal of each subgenus is furnished 
with a proboscis, long approximated tentacula, on the external 
side of which are the eyes, and with a horny operculum: the 
_veil on the head is wanting; and, the length of the siphon ex- 
cepted, it otherwise resembles that of the buccina. 
To this genus Linneus has added several purpure, in 
which the canal is not salient, and all the cerithia, in which it 
is recurved. 
Bruguiéres divides them into two genera, which have been 
since subdivided by Messrs. Lamarck and Montfort. 
MuREX, Brug., 
Includes all those which have a salient and straight canal, 
with varices across the whorls. These varices are projecting 
rims, with which the animal edges its mantle, at each inter- 
ruption in the growth of its shell. 
