488 PHILOSOPHY OF ZOOLOGY. 



A. Aperture of the shell entire. 



As co-existent with this character of the shell, the ante- 

 rior margin of the cloak, at the entrance to the branchial 

 cavity, is found likewise to be entire. 



1 . Aperture of the shell closed by a pedal lid, or oper- 

 culum. 



The three following families appear to be the indications 

 of as many natural groups, the genera of which admit of 

 still more minute arrangement. 



Family 1. Turbonida. Aperture of the shell round or 

 ovate. 



This family includes the greater number of the species 

 of the Linnaean genus Turbo. The genera into which it is 

 now divided may be distributed into two sections, from the 

 residence of the animals. 



Section 1. Marine. 



The marine Turbonidge are of frequent occurrence, and 

 compose the genera Turbo, Delphinula, Turritella, Scalaria, 

 Odostomia, Monodonta, Phasianella, and Vermicularia. 

 Some of the species are known to be ovoviviparous, and it 

 is probable that the same kind of reproduction prevails in 

 all of them. Remarkable differences may be observed in 

 the form of the hood, the length of the peduncles support- 

 ing the eyes, and the number and distribution of the fila- 

 ments surrounding the body' 



Section & Fluviatile. 



The Fluviatile Turbonidae are limited in thrir number, 

 both in regard to genera and species. The genera are only 

 three, Ampullaria, Melania, and Paludina. In the last of 

 these, including the Helix vivipara, and tentaculata, the 

 sexes are obviously distinct in different individuals. 



Family 2. Neritada. Aperture semicircular, with an 

 oblique, straight pi]lar-]:p. 



