With 



») 



VOL. II.] Studies Among Mollusks. 315. 



the genus Selenites, as we understand those genera at the present 



time. 



In the Manual of American Land Shells, Mr. Binney describes 



the animal of the family Selenites " Without a caudal mucus pore, 

 jaw of Limacida;." In the family Limacidae he includes Limax, 

 Vitrina, Zonites and Vitrozonites. The animal of this family is de- 

 scribed, "With or without a caudal mucus pore, jaw arched with- 

 out ribs." In the family Helicidae he includes twenty-seven 

 genera. The animal of this family is describe 

 out a caudal mucus pore, jaw of many patterns. 



Now it will be seen by this that the caudal mucus pore is not an 

 invariable character, for 'it is both present and absent in the two 

 families of Limacidae and Hehcidae, therefore we cannot use it as a 



generic character. 



The smooth or ribless jaw is found in the three families of Sele- 

 nitidie, Limacida and Pupidae, while the jaw of the animal com- 

 prising the family Philomycidse is described, "With or without 

 anterior ribs." The ribbed jaw of the Helicidae is known to be so 

 variable, even in the same species, that it is hardly worth referring 

 to for this purpose. The jaw then offers us no characters. Hues or 

 limits, that can be relied upon to distinguish or determine genera. 



Some students attach great importance to the form of the lingual 

 membrane, and the arrangement of the teeth, and think they have 

 found the keystone to the systematic arrangement of mollusks in 

 these useful organs, which are used in the economy of the animal 

 simply to chop or grind up the food, for its reception in the stomach 

 preparatory to the process of digestion. Now I do not underes- 

 timate the value of the knowledge of the structure, form and the 

 uses, as well as of the varied and beautiful arrangement of those 

 organs, but it seems to me a system based on the dentition alone 

 would represent the carnivorous and herbivorous habits of the 



animal and nothing more. 



For the purpose of determining genera the lingual membrane 

 rather adds to the confusion than otherwise, as will be seen by the 

 following list of families that have similar lingual membranes, which 

 I have selected from Mr. Binney' s Manual of American Land 



Shells: 



Family Selenitidse membrane of Testacellidae. 



Family Philomycidse membrane of Helicidae. 



