52 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



more graceful. It must constantly be borne in mind that on any one 

 membrane the teeth vary considerably in regard to this point. 



In illustrating the general arrangement of the teeth upon the lingual 

 membrane, I have used the wood-cuts in the text prepared for my 

 former works and papers, mostly by Mr. Morse, and a few by Dr. Leidy, 

 prepared for my father's work. It must be remembered that these 

 figures do not represent correctly the characters of the individual 

 teeth. 



I have also used in the text figures of the 'jaws of many genera and 

 subgenera, prepared for the Land and Fresh-Water Shells of North 

 America, Part I. The jaws of the more recently described genera and 

 subgenera I have myself illustrated from drawings by camera lucida. 



'On the Value of the Jaw and Lingual Membrane for the Purpose 



of Classification. 



It is conceded by all recent students of land shells that for the larger 

 divisions the presence or absence of a jaw and the aculeate or quadrate 

 form of marginal teeth are reliable characters. 



The characters of the jaw and separate teeth of the lingual membrane 

 have also been used in various ways for grouping the genera into fami- 

 lies, etc., and even of grouping species into genera. I refrain from any 

 discussion of their value for such purposes, simply because I believe 

 our material is far too limited. It seems as if I can better employ my 

 time in patiently accumulating new facts. I can, however, venture to 

 say that the character of the jaw and teeth seems to be more constant 

 in some genera than in others. It appears, for instance, that in some 

 genera the presence or absence of lateral teeth is not a generic character, 

 though in others it is. The same may be said of the presence or ab- 

 sence of side cutting points to the centrals and laterals, and the greater 

 or less development of their side cusps ; also in the bifurcation or non- 

 bifurcation of the cutting point of aculeate marginal teeth ; also as to 

 the presence or absence of ribs on the jaw. 



It will, I believe, be proved that certain genera are constantly char- 

 acterized by a peculiar form of teeth, while others have a considerable 

 range of variation. I might, perhaps, add that when the genus is 

 numerous in species, there is a much greater chance of finding a varying 

 dentition. If this latter proves true, we shall be obliged to concede 

 that there are certain types of teeth which may be found among species 

 of some of the larger genera, though some of the smaller genera are 



