86 CLASS GASTEROPODA. 
the most common species, round the foot, and on to the mouth. 
Outside its long tentacula are two cylindrical pedicles, which 
support the eyes. The mantle is deeply cleft on the right side, 
and the water, which passes through the holes of the shell, 
penetrates through this cleft into the branchial cavity. Along 
its edges we observe three or four filaments, which the animal 
can protrude through these holes. ‘The mouth is a short pro- 
boscis. All the halyotides of Gmelin, except ¢mperforata and: 
perversa. This genus has its counterpart among the fossils. 
The PapoLu®, Montfort, have a circular shell, in which 
the holes are nearly obliterated ; and there is a deep furrow 
that follows the middle of the whorls, and is marked exter- 
nally by a salient ridge. 
STROMATIA, Lam. 
The shell more hollow, the spire more salient, and the 
holes wanting; otherwise, like that of the halyotides, which 
it thus connects with certain species of turbo. The animal is 
much less ornamented than that of halyotis. 
In the following genera, which are separated from the patellz, 
the shell is perfectly symmetrical, as well as the position of 
the heart and branchie. In 
FISSURELLA, Lam., 
We perceive a broad fleshy disk under the belly, as in the 
patella, a conical shell placed on the middle of the back, but 
not always completely covering it, and perforated at its sum- 
mit by a small orifice, which affords at once an issue for the 
feeces and a passage to the water, required for respiration ; this 
orifice penetrates into the cavity of the branchie, situated on 
the fore part of the back, and in the bottom of which ter- 
minates the anus; a cavity otherwise widely opened above the 
head. A branchial comb is symmetrically arranged on each 
side; the eyes are on the external base of the conical tenta- 
13 
