NATURAL SCIENCES OF PHILADELPHIA. 253 
Helix. Thus we have found the jaw alike in all the species we 
have examined of Patula. So of Sagda, Polygyra, Mesodon, 
Arionta, Stenotrema, Triodopsis, Plagioptycha, Polymita, Denitel- 
larva. 
It appears to us, therefore, that we may hope to find a reliable 
generic character in the jaw, when the present subgenera, or some 
of them at least, are recognized as distinct genera. On account 
of the gradations in the ribs and median projection, we have else- 
where' suggested that for the purpose of grouping the genera into 
subfamilies, we may depend only upon the following distinctions 
in the jaw. 
(a.) Jaw in one piece. 
(b.) Jaw in separated, imbricated pieces. 
(c.) Jaw in one piece with an upper accessory plate. 
As regards the lingual dentition, it appears that the distinction, 
as far as the central and lateral teeth are concerned, is but slight 
between the various families and genera. The marginal teeth, 
however, give us two very distinct types, the aculeate and quadrate. 
The former is found either unaccompanied by a jaw, or by its 
simplest form. Up to this date no instance is on record of aculeate 
marginal teeth together with a jaw complicated by anterior ribs, 
free imbricated plates, or an upper accessory plate. The presence 
of aculeate marginal teeth is usually (not always) indicated by a 
smooth, shining shell, with acute peristome. The shell, however, 
is not always reliable as an indicator of the form of marginal teeth, 
for we have shown the rough shells of several Mauritius species 
to have a lingual membrane with aculeate marginal teeth. It 
appears, therefore, that the shell alone will not indicate the generic 
position of some species. 
The quadrate marginal teeth are accompanied by various forms 
of jaw. They are usually constant in shape in the various genera. 
Sometimes, however, we have met with unexpected variation— 
such as in Macroceramus Gossei, Helin muscarum, and a group 
of Bulimulus represented by B. lalicinctus. 
The form of jaw with free, imbricated plates seems (excepting 
in Punctum) constantly accompanied by a peculiar form of quadrate 
teeth, as in Orthalicus and Liguus. To our surprise, however, 
we have detected a somewhat similar form of teeth in Geoiis, 
1 Ann, Lye. N. H. of N. Y., X. 168. 
