A MANUAL OF AMERICAN LAND SHELLS. 49 



of the greatest expansion often sliows a lateral bulging representing the 

 side cutting points in species deprived of distinct side cutting points. 



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

 as in my figure, api^ears 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 Fig. 6), differs in not 

 having a quadrate base of attachment, but usually one fig. e. 



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 some- 

 times more spine-shaped. The apex of the cutting 

 point is sometimes bifid, or even trifid, even in the First marginal tooth 



of Zonites inornatus. 



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. 



In the fifth volume of Terr. Moll. U. S., I have given a figure of the 

 dentition of each species which I have examined. A reference to this 

 figure is given in the descriptive part of this work. Under each genus 

 are given figures necessary to illustrate the dentition of the genus. I 

 give, however, figures here of the most usual types of dentition found 

 in the genera furnished with quadrate marginal teeth. 



(a) Lingual membranes with no side cusps or cutting points on any 

 of the teeth, even the extreme marginals, are rarely, if ever, so found 

 The nearest approach to this is in Mesodon ^i° '^• 



thyroides, Wheatlei/i, and clausus. I have r~7() /O y^ /A f^ 

 figured that of thyroides. It will be ob- /]4\uA A4 ^ ^ ^ 

 served that the extreme tooth at the right ^^ (/ 



has its cutting point bifid, and has a small Lmguai dentition of ju<;«odo,wfti/roidf«. 

 side cutting point. 



(h) The next form of dentition has the ^^^' ^' 



central and first laterals without developed ^-. >;--, ^^ f-T^fv?! 



side cusps or any cutting points, the outer 30 ^-^ ^^ rJ \ ^^^^-^ 

 laterals and marginals with them, such is 12. 7 i 



Linscual dentition of Patwto Cumber- 



Patula Cumherlandiana, here figured. landtana. * 



* The numbers indicate the position of the teeth from the central line of the mem- 

 brane. 



1749— Bull. 28 4 



