90 HELICID^. 



■(Then the anatomical structure and embrjological devel- 

 opment of the animal shall have been more fully investi- 

 gated, such divisions -will be made. The investigations 

 already made by Drs. Leidy and Wyman into the ana- 

 tomical details of the snails, have not only made it possi- 

 ble to determine the limits of species which could not be 

 satisfactorily settled by the character of the shell alone, 

 but have shoivn that there are distinct and various types 

 in the structure of the animals ; and that for a group of 

 allied shells the animals belonging to them have a peculiar 

 and allied structure also. In confirmation of this we need 

 only to refer to the cuts of the denticles on the tongue of 

 S.profanda, solitaria, and multilineata,oi M. albolabris 

 and thyroidiis, of H. tridentata and auriculata, and of 

 jBT. jndehella and lineata, to see that there is no less a 

 correspondence in the forms of the denticles, in these 

 respective groups, than in the conformation of the shells 

 themselves. This is still fui'ther evident if we inspect 

 also the viscera, especially the generative system, as 

 seen in the instances of M. ligera and intertexta, and H. 

 coneava, perspectiva, solitaria, and aUernata, two groups 

 of shells in each one of which there is a pecuUar con- 

 formation of the viscera, quite different in one from what 

 we find it in the other. 



Without attempting to indicate the natural relations 

 of all the species to each other, a few of the principal 

 groups may be noted, in which the species named are 

 evidently allied to each other, though the grouping does 

 not correspond to any arrangement hitherto made. 



