NATURAL SCIENCES OF PHILADELPHIA. 15] 
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 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 may 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 type 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 
