FREE INSTITUTE OF SCIENCE 



21 

 MASTODON 



jaw. Thus, the third milk molar of the right side is rather more worn than 

 that of the left side; the first right premolar is worn at the summit of the 

 largest or first outer lobe, while the left slightly larger tooth is entirely 

 unworn ; and the left first true molar is most abraded at the summit of the 

 first outer lobe and slightly on the contiguous inner lobe, while in the left 

 tooth less abrasion appears only on the first outer lobe. 



Plate iv. fig. 10 represents the first premolar and the succeeding third 

 milk molar of the left side ; plate ii. fig. 2 represents the first true molar of 

 the right side, better preserved than the other; and plate iv. figs. 9 and 11 

 represents the second premolar from the same specimen. 



The first premolar tooth has an ovoid crown, widest behind, and is 

 situated obliquely in its position in relation with the line of the teeth behind. 

 The fore and aft diameter inclines inward and forward from that of the suc- 

 ceeding second milk molar with which it is in contact. The tooth of the left 

 side is slightly the larger, and as before indicated, is entirely unworn ; which 

 seems the more remarkable from the apparent greater completeness of its 

 investion, for the right tooth yet occupies a larger excavation, left by its pre- 

 decessor, the first milk tooth. 



In the third milk molar, plate iv. fig. 10, the summits of all the constituent 

 lobes of the crown are worn away so as to expose conspicuous dentinal tracts. 

 The inner lobes are most worn and exhibit broad shield-shaped dentinal 

 excavations, which are variably trilobate medially, according to the extent of 

 the offsets of the lobes in this position. The dentinal tracts of the outer lobes 

 are much smaller and are most extended transversely. Their shape conforms 

 to the wear of a double eminence. On the first outer lobe are islets side by 

 side ; on the second the tract is transversely elliptical and constricted at the 

 middle, and on the third lobe it is transverse elliptical and bent. In the 

 opposite tooth all three outer tracts are more worn, are transversely elliptical 

 and constricted at the middle. The tooth has a strong basal ridge internally 

 but somewhat interrupted at the middle of the corresponding lobes. 



The first true molar, plate ii. fig. 2, is typical of the usual form and con- 

 struction of all the molars of the species from the first to the last, except the 

 premolars. The inner constituent lobes of the crown arc all medially extended 

 both fore and aft in stout tubercular buttresses, of which those contiguous 

 unite in the intermediate transverse valleys, while that in front of the crown is 

 the most robust and is continued in advance of the contiguous outer lobe, and 

 that at the back of the crown is more isolated than the rest. The outer lobes, 



