260 Annals of the Carnegie Museum. 



fixed in the jaw by only one root. The crown is composed of two 

 cusps, one anterior, the other posterior. The anterior is the larger of 

 the two. This tooth exhibits only faint traces of wear, and its erup- 

 tion must have been somewhat subsequent to that of d. m.i. Decidu- 

 ous m.2 is (juite molariform, has a very long crown and was fixed in 

 the jaw by two widely separated roots, one anterior and the other 

 posterior. The crown is much worn and anteriorly it is produced 

 somewhat beyond the anterior cusps and forms an additional anterior 

 grinding surface, not seen in the succeeding lower milk molars, nor in 

 the replacing permanent teeth. It was this portion of this tooth that 

 in life was opposed to d. m.i and accomplished the erosion already 

 noticed on the crown of that tooth. Deciduous m.^ is only a little 

 less worn than the preceding tooth and is strictly molariform. De- 

 ciduous m.j is much larger and less worn than are the preceding teeth. 

 It is in form like permanent molars y and -^ and has not the third lobe 

 seen in the last lower molar of all the Titanotheres. The absence of 

 this lobe in this tooth is somewhat remarkable and would seem to form 

 an exception to the general rule f/nr/ tJie last deciduous loiuer molar in 

 the ungulates has the form of the last permanent lower molar, rather 

 than that of the fourth premolar, wliich replaces it. In the present in- 

 stance this tooth is like the fourth premolar and ms. j- and ,j, and this 

 rule would seem to hold good in the genus Titanotherium, for I have 

 recently examined a number of other jaws of young Titanotheres and 

 find in each instance the same conditions. 



The Permanent Dentition. 



Of the permanent dentition in the skull under discussion only 

 ms.^ are erupted and in position. These teeth are in either instance 

 slightly worn, while ms.fare already formed and in position ready 

 for eruption. But ms.f are only just commencing to form. The 

 cusps are partially developed, but in each instance they are separated, 

 not having assumed their proper positions in the jaw. Each cusp 

 seems to be developing independently of the others and from different 

 centers, much as certain bones ossify from different centers, which 

 later become connected and form a single bone or tooth as the case 

 may be. In the lower jaw the molars are first formed high up in the 

 ascending ramus just beneath the condyle and base of the coronoid 

 process. From this position, as the jaw lengthens with the growth of 

 the animal, the tooth moves downward and the crown assumes a ver- 



