DAKOTA AND NEBRASKA. 241 



have their exterior sides more sloping and their summits usually narrower than the 

 other lobes. These have usually longer and wider summits, and generally present a 

 notch and groove more or less distinct, apparently indicating a tendency to sub- 

 division into a pair of lobules, or, in other words, the inner lobes of the lower molars 

 and the outer lobes of the upper molars appear to be composed of a connate pair of 

 lobules more or less evident. 



The outer lobes of the lower molars, and the inner ones of the teeth above, present 

 a buttress-like sub-acute ridge iu front and behind, extending from the summit to the 

 bottom of the intervening valleys. The lobes just indicated rather obscurely exhibit 

 an apparent composition of a connate pair of lobules. Sometimes the included lobule 

 presents ridges apparently coiTespouding with the buttress-like ridges of the 

 embracing lobule. Likewi.se among the longitudinal ridges, frequently occupying the 

 sloping surfaces of the other lobes, a pair may often be observed corresponding with 

 the buttress-lilce pair of the former lobes. 



The third division of the crown of the second molar is constituted like the others, 

 but is more or less rudimental. Its external lobe in the lower tooth, and internal 

 one in the upper tooth, are the least developed and sometimes appear nearly obsolete. 



The heel of the last molar, usually less pronounced in the upper than the lower 

 tooth, consists of the thick contiguous element of the basal ridge, sometimes alone, but 

 oftener compounded with one or more conical eminences more or less well-developed, 

 and approaching in character a fifth ridge or division to the crown. 



As the molars of M. americanus are worn away, more or less lozenge-shaped tracts 

 of dentine appear at the summits of the inner lobes of the teeth above and the outer 

 lobes of those below ; and transversely oval or clavate tracts, occasionally slightly 

 lozenge-shaped, appear on the other lobes. 



Considerable difference exists between the corresponding teeth of different individu- 

 als of 31. americanus. They vary in size, in proportions, in degree of development of 

 constituent elements of a basal ridge at the outlets of the transverse valleys of the 

 crown, in smoothness of the enamel, etc. Lateral constituents of the basal ridge may 

 be obsolete, moderately developed and smooth, or well marked and rugged. Differ- 

 ence in degree of development of the buttresses of the inner lobes of the upper teeth, 

 and of the outer lobes of the lower teeth, produce a variation in the freedom of ob- 

 struction to the course of the transverse valleys. The last molar tooth is especially 

 of a variable character, differing much in size and in the proportion of its diameters. 

 Generally possessing four transverse ridges and a heel to the crown, occasionally there 

 are five ridges with or without a heel, and sometimes the number is reduced to four 

 without a heel. With a diminution in the number of ridges the teeth usually are of 

 more uniform width ; with the increase there is usually a proportionate degree of 



narrowing of the crown posteriorly. 



31 



