TODOPHTHALMA. 79 



mouth, without any canal in front. They are pro- 

 vided with an operculum, which covers the mouth of 

 the shell upon the animal, and which, from its position 

 and the manner of its formation being similar to 

 that of the shell, may be considered as a free second 

 on valve. 



This order is divided into groups, by the position 

 of the eyes, and the absence or presence of tentacles 

 on the side of the body. 



Sect. I. Podophthalma 



Have the eyes placed on a separate pedicel at the 

 hinder edge of the tentacles : the heart is generally 

 traversed by the rectum, as in the Conchifera. 



Fam. 1. NeritidcB. The sides of the body simple: 

 shell ovate; mouth half-ovate, with an acute inner 

 lip; operculum appendaged. (f. 1 3.) 



Sect II. Eriophthalma. 



Eyes of the animal sessile at the base of the tentacles : 

 dioecious. 



Fam. 2. Melaniadce, animal ; trunk elongate : shell 

 ovate ; mouth ovate, not continued ; operculum 

 horny, ovate, spiral, (f. 4 6.) 



Fam. 3. Paludinidce, animal ; gills enclosed : shell 

 conical ; mouth ovate, continued ; operculum an- 

 nular, (f. 711.) 



Fam. 4. Valvatida, animal ; gills exserted : shell coni- 

 cal ; mouth round, continued ; operculum horny, 

 orbicular, many-whorled. (f, 12 14.) 



E 4 



