CHAP. III. TRIBES OF THE GASTEROPODA. 59 



interesting^ because it contains those diverging genera 

 which connect it not only with the chitons^ but with 

 the Gasteropoda on one side, and the Cephalopoda, on 

 the other. They may be described as thick, fleshy, 

 soft mollusks, generally possessing a distinct head 

 furnished with a pair of ear-shaped tentacula, and with 

 the mantle usually dilated into two lobes resembling 

 fins : the branchia are folliculated on the right side or 

 on the back, and are generally covered by a small 

 bulla-shaped shell concealed in the folds of the body. 

 They have the power both of crawling by means of a 

 narrow disk on their belly, and of swimming by their 

 fins. JNIany of the species are very large, and when 

 taken out of the water they appear like great oval 

 masses of flesh. They are the only swimming gas- 

 tropods possessed of a univalve shell, and are con- 

 nected to the Cephalopoda by the singular genus Gas- 

 teropteron Meek., which thus brings us at once to the 

 Pteropoda ; while the connection of the Bullce to the 

 CypraidcR is obvious to every one. 



(52.) The union of the Phytophaga and the Zoophaga, 

 or the typical divisions, is so perfect, that the only ap- 

 parent difliculty seems to be where one terminates and 

 the other begins : thus, if we look to the possession of a 

 siphon as a primary distinction of the Zoophaga, we 

 find this organ fully developed in the sub-aquatic genus 

 Ampullaria, which is nevertheless so closely united in 

 its other characters to the Phytophaga, that we cannot 

 separate it from its obvious allies : the mouth of the 

 shell, in fact, is as entire, or rather as destitute of any 

 notch or canal, as that of the garden snail ; while PlO' 

 naxis and Melanopsis, both of which are most inti- 

 mately related to Melania, have a deeply notched 

 aperture. If we look to the relations afforded by the 

 animals themselves, the same interchange of characters 

 takes place. As these two groups follow each other in 

 afl5nity, we may pass on to the HelicidcE, or land snails, 

 and the marine Trochidce, until we reach the Scuti- 

 hranchia by means of such genera as Trochella, which 



