54 CLASS GASTEROPODA. 
from each other by the filaments, fringes, and other ornaments 
of the head, foot, or mantle. 
These mollusca are arranged in several families, according 
to the forms of their shells, which appear to be constantly in 
relation to that of the animal. 
The first family of the gasteropoda pectinibranchiata, or the 
TROCHOIDA, 
Is known by the shell, the aperture of which is entire, with- 
out an emargination or canal for a siphon of the mantle, as the 
animal has none, and is furnished with an operculum or some 
organ in place of it. 
Trocuus, Linn., 
Have shells; in which the external margin of the angular 
aperture approaches more or less to a perfect quadrangular 
figure, and is in an oblique plane with respect to the axis of 
the shell, because the part of the margin next to the spire 
projects more than the rest. Most of these animals have 
three filaments on each edge of the mantle, or at least some 
appendages to the sides of the feet. 
Of those which have no umbilicus, there are some in which 
the columella, having the form of a concave arch, is continu- 
ous with the external margin without any projection. It is 
the angle and projection of this margin which distinguish them 
from turbo. These are the TEcTARIA, Montf.; Troch. iner- 
mis, Chemnitz, &c. 
Several are flattened with a trenchant edge, which has 
caused them to be compared to the rowel of a spur. (CAL- 
cAR, Montf.; Turbo calcar, L., &c.) 
Some, again, are slightly depressed, orbicular, and shining, 
with ahalf-rcunded aperture, the columella convex and callous. 
(ROTELLA, Lam.; Troch. vestiarius, L.) 
The columella of others is distinguished near the base by a 
