natural sciences of philadelphia. 151 



On the Value op 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 families, 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 ma3 r be said of the presence or absence of side cut- 

 ting 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. 



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

 characterized by peculiar form of teeth, while others have a con- 

 siderable 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 much more, if not abso- 

 lutely, restricted to one single t} r pe of dentition. I do not ven- 

 ture any further deductions at this time. 



Before closing my paper I must return thanks to my many cor- 

 respondents, who have furnished me specimens for examination 

 during many years. I have already acknowledged their kindness 

 while originally describing the jaw and lingual membrane of each 

 species in the American Journal of Conchology, the Annals of the 

 Lyceum of Natural History of New York, and the Proceedings 

 of the Academy of Natural Sciences of Philadelphia. Most of 



