434 MEMOIRS OF THE CARNEGIE MUSEUM. 



lighter zygomatic arches without the heavy rugosities on the posterior angle, and 

 the less everted angle of the lower jaws. In connection with these characters 

 the skeletal frame is lighter and the pelvic cavity proportionally somewhat larger, 

 judging from the material at hand. 



Mandible. 

 (PL LXVI, Figs. 2 and 4.) 



As stated elsewhere-" the lower jaws are heavy, especially those of the males. 

 The depth of the ramus, however, is not great, and the diastema between the lateral 

 incisor and P2 is rather short. The symphysis, though short, is very heavy, and 

 the median suture is entirely obliterated in old individuals. The angle is very 

 greatly everted; in males the border of this everted area is very rugose, while in 

 females and young the angle is less everted and is also less rugose, but still furnishes 

 an unusual heavy surface for the masseteric muscles. The glenoid condyle is quite 

 broad transversely and the coronoid process is strongly directed forward. 



Dentition. — In proportion the median incisor is extremely small; it has a 

 rounded enamel-covered crown, and is implanted in the symphysis by a thick 

 short root; in many specimens this rudimentary tooth has dropped out. The 

 lateral, or cutting incisor, is, as usual, comparatively large, but varying in different 

 individuals. The canine is generally absent, even in the young. Occasionally 

 this tooth is present in young individuals, and sometimes there is found on the 

 alveolar border of the diastema a shallow groove, or scar, in which a minute canine 

 is found in a very procumbent position. Pi is absent. In young individuals a 

 small milk-tooth is often found immediately in front of Po which persists in the 

 alveolar border until all the cheek-teeth are erupted. On PI. LXVI, Figs. 2 and 4 

 the characters of the cheek-teeth are well shown, and require no detailed de- 

 scription. 



Only a very few hyoid bones are found mixed with the general mass of ma- 

 terial in the quarries, there evidently having been during deposition too much 

 disturbance for the preservation of these delicate parts. 



In this connection it is interesting to turn to the work of Professor Henry 

 F. Osborn, "The Extinct Rhinoceroses" Mem. Amer. Museum Natural History, 

 Vol. I, 1898, pp. 136-140. In this publication, Osborn characterizes Ccenopus 

 {Aceratherium) mitis from the lower Oligocene as follows (p. 139): "Ci", P3, M3, 

 Diastema short. Canine" alveoli semi-procumbent. Premolar-molar series 



" Peterson, 0. A., Ann. Car. Mus., Vol. VII, 1910, p. 275. 

 " [= Lateral incisor.] 



