NATURAL SCIENCES OF PIIILADELPIITA. 253 



Helix. Thus we have found the jaw alike in all the species we 

 have examined of P alula. So of Sagcla, Pohjgyra^ Mesodon^ 

 Arionta, Slenotrema^ Triodopsis, Plagioplycha^ Polymita, Dentel- 

 laria. 



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. 



(&.) 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 bj' 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 mai'ginal 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 b}' 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 3Iacroceramus Gossei, Helix muscarum, and a group 

 of Bulimulus represented by B. lalicincfui^. 



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 Gseotis, 



' Aun. Lye. N. H. of N. Y., X. 163. 



