GASTEROPODA. 169 
and to show also, that their dividing lines do not coincide with any of 
the artificial limits hitherto drawn from the shell alone. We are dis- 
posed to place more value on the structure of the operculum than M. 
Deshayes is inclined to. According to our observations, a modification 
of the operculum is found to be accompanied by a decided modification 
of the animal, much more certainly than a variation in the form of the 
shell is. The number or absence of the lateral cirrhi also, is a cha- 
racter to which we should give some weight, on the observed prin- 
ciple, that multiplication of parts performing the same office indicates 
inferiority of rank. 
As no summary of the characters of the animal of the Trochide has 
been given, we have made the following abstract of them, so far as they 
relate to external configuration. 
Anima short; locomotive disk truncate anteriorly, broadly rounded 
posteriorly ; head proboscidiform, without trunk; tentacles long, fili- 
form, annulate ; eye pedicles distinct, generally bearing a cartilaginous 
auricle at the inner base, beneath which the tentacle issues: a lateral 
veil passes backwards from the adhering muscle of the animal to the 
operculum, its margin undulated and more or less incised, and usually 
bearing cirrhi. 
As in this work we are restricted to the consideration of individuals, 
rather than at liberty to enter upon more general propositions, we 
have arranged our species under such generic names as are commonly 
received. 
TuRBO sETosUS (GMELIN), CHEMNITz, Conch., v. 181, f. 1795, 1796. 
Lamarck, An. sans Vert., ix. 191, &c. 
Antmat., Head prolonged into a broad, sub-trilobate muzzle, flat- 
tened out in front, and having a duplication at its inferior margin, con- 
tinued from the buccal fissure in a deep sinus terminating at the junc- 
ture of the foot and the body near the peduncle of the right eye; on 
the under side of this peduncle, at about one-third the distance from 
the base, is a small mammilliform tubercle. The foot is broadly oval 
with a profound sinus down the middle, so that the anterior margin is 
indented by it; on each side of this sinus are numerous large, transverse 
43 
