5 8 Bulletin American Museum of Natural History. [Vol. X, 



inal girth exceeds that of R. unicornis, justifying Cope's conclusion 

 that this animal had rather the proportions of the Hippopotamus 

 than of the Rhinoceros. It will be recalled' that R. unicornis has 

 a lower abdominal line than R. sondaicus or R. smtiatrensis, or than 

 either of the African Rhinoceroses. T. fossiger, therefore, had a 

 totally different external appearance from any existing form. 



Juvenile, Senile and Sexual Characters. 



There are conspicuous differences in the dentition of different 

 specimens, all of which may be explained as due to influences of 

 growth or sex. 



In the young calf jaw (No. 2608) the milk cutting teeth are as 

 follows : di^, dcy, dp 3. In the young R. sondaicus {R.javanicus), 

 according to de Blainville, we similarly observe two milk incisors. 

 A somewhat older calf of T. fossiger shows diy, dcy, dp^. 



Even in older jaws there is evidence in one case (No. 8391) of 

 two lower incisors upon one side, the formula being : ij-j, cj, Y>^, 

 m-j, as indicated by the incisor alveoli. The outer incisors (ig) in 

 the lower jaw tend to drop out "at an early age, leaving only the 

 alveoli ; but the vestigial upper incisors (i^) are remarkably tena- 

 cious, although entirely useless. 



The canines vary strikingly in the sexes. In the females (Nos. 

 2604-6, 2610-11, 2623), as shown in the photographs, they are of 

 moderate size. In certain males (Phillips Co., Kansas, No. 2612 ; 

 Republican River, Nebraska, Nos. 8391-2) they exceed in size any 

 that have been recorded in other Rhinoceroses living or extinct, 

 as shown in PI. IVa. 



No jaw shows any vestige of pj. Pg is present in young jaws, 

 and invariably absent in very aged jaws. Its dehiscence is cor- 

 related with the coming into use of mf. The upper molars, 

 especially the second and third, are extremely hypsodont, the un- 

 worn enamel of the crown measuring, respectively : m2, = m3 = . 

 They are reserved for middle and old age. 



The size of the skull differs considerably in the two sexes, the 

 female skulls (Phillips Co., Kansas, Nos. 2604, 2607, 2622-3 '■> ^^- 

 catur Co., Kansas, No. 8388 ; Republican River, Nebraska, No. 



' See Sclater 'On the Rhinoceroses now or lately living in the Society's Menagerie,' Trans. 

 Zool. Soc, 1875, pp. 645-651. 



