THE JAW AND LINGUAL MEMBRANE. 47 



point is flat on its lower surface, that is, the surface nearer the base of 

 attachment, but from thence it first rises and expands greatly at its 

 sides, and then gradually decreases in size as it still rises pig 5^ 

 and arches over the top. Thus under tlie microscope there 

 are two planes prominently seen by changing the focus of 

 the instrument, the plane of the lowest portion of the cut- 

 ting point, and the plane of its gi'eatest expansion. In 

 Fig. 5 the former is shown by dotted lines, the latter by 

 the continuous line. In my illustrations the former alone 

 is given. I regret not having shown both as done by Sem- 

 per in Phil. Archip. 1. c, especially as the plane of ^^^ riTHnaterKi of z. 

 greatest expansion often shows a lateral bulging represent- /"^'e^nosua. 

 ing the side cutting points in species deprived of distinct side cutting 

 points. 



The median cutting point seen on the plane of its greatest expansion, 

 as in my figure, appears to spring from the median cusp itself, as if it 

 were not distinct from it. A great deal has still to be done in eluci- 

 dating the true character of cusp and cutting point. 



The other type of tooth, which I call aculeate (see Glandina), differs 

 in not having a quadrate base of attachment, but usually one of a 

 somewhat sole-like form. Its upper margin is not reflected, but from 

 its whole surface springs a single large cutting point, usually thorn- 

 shaped, but sometimes more spine-shaped. The apex of the cutting 

 point is sometimes bifid, or even trifid, even in the same genus. 



Of these two types, quadrate and aculeate are all the teeth now 

 known. Of the quadrate type many and dissimilar forms are known, 

 but all have the quadrate base of attachment. 



The characteristics of central, lateral, and marginal teeth are given 

 under each genus or subgenus. 



On Classification. 



The characters of the jaw, combined with those of the lingual mem- 

 brane, furnish reliable bases of classification. They have been consid- 

 ered of various weight by different writers. I here propose to treat 

 them as guides only to the greater division of the Pulmonata.^ In 

 grouping the genera it will be necessary to include all, both native and 

 foreign to America, in order to properly appreciate the value of this 

 arrangement. 



1 I must not be understood to propose a system of classification. I merely place the 

 genera into certain groups, independent of their divisions into families. 



