146 ! MOLLUSCA. 



difications of.fleshy lips and corneous jaws. The inside oi 

 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 base 

 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 as a tongue, in propor- 

 tion as the anterior part is worn by use and absorbed. (See 

 his Memoir e sur la Vivipare tfeau douce, p. 12; and Mem. 

 sur laPatelle, 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 Pulmones; 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- 



