146 ~~“ MOLLUSCA. 
difications of fleshy lips and corneous jaws. The inside of 
the cheeks are covered in some species with reflected teeth, 
to aid deglutition. The tongue can scarcely be detected in 
some of the genera; while, in others, it is a simple tubercle, 
or a strap-shaped, spiral organ, armed with transverse rows 
of teeth. This spiral tongue, where it is fixed to the kase 
of the mouth, is broadest, and there also the spinous pro- 
cesses are strongest. The spiral part is narrowest and soft- 
est, and folded up behind the pharynx. M. Cuvier conjec- 
tures, and apparently with plausibility, that the spiral portion 
comes forward into the mouth to act asa tongue, in propor- 
tion as the anterior part is worn by use and absorbed. (See 
his Mémoire sur la Vivipare @eau douce, p. 12; and Mém. 
sur la Patelle, p. 17). 
The organs of respiration exhibit the two modifications 
of lungs and gills, to enable us to divide the Gasteropoda 
into two classes, which we have termed Pulmonifera and 
Branchifera. M. Cuvier appears to have been in some mea- 
sure aware of the importance of the distinction, when he 
instituted his order Pudmonés; but he afterwards suffered 
himself to be more influenced by the presence of an oper- 
culum, the shape of the aperture of the shell, and the ‘sup- 
posed separation of the sexes, than by the characters of the 
respiratory organs. 
Some shells are simply tubular or conical; but the greater 
part are variously convoluted, the volutions being termed 
whorls or spires. These whorls are in general visible and 
distinct, the boundary between each being termed the Line 
of separation. The whorls in some species are simply placed 
in a lateral position, while in others the whorls are formed 
upon a pillar, or columella, which runs in the direction of 
_the axis of the shell, the inferior whorl in this case embrac- 
