48 E. L. Troxell — Ccenopus, the Ancestral Rhinoceros. 



ilarity to the skull of C. proavitus Hatcher in the form of 

 the wide area between the parietal ridges on the sagittal 

 crest. These ridges for a considerable distance are almost 

 straight, converging at an angle of about 30° up to a 

 point 8 or 9 cm. from the occiput, whence they run uni- 

 formly parallel. The occiput rises well above the plane 

 of the face. 



Both the proto- and metalophs are straight ; the latter 

 are especially thin and the terminating* cones are not 

 intimately connected with the strong postero-internal 

 cingulum. The metaloph does not increase the length 

 of its surface with wear, but with advanced age it may 

 unite with the protoloph, at a point, however, scarcely 

 more than halfway along the latter. The tetartocone, 

 though small, is not recessive; it stands near the outer 

 edge of the tooth crown, thus giving the great length to 

 the metaloph. 



The deuterocone, as it terminates the protoloph, swings 

 forward, away from the tetartocone, in decided contrast 

 to the C. trigonodus or C. platycephalus type of premolar, 

 in which it curves backward, envelops the central part of 

 the tooth and sometimes even the metaloph itself. The 

 cingula.are interrupted. for short distances on the proto- 

 cones and hypocones of the first and second molars, but 

 the one is continuous on the protocone of the third molar. 

 On P^ there is a small crista. The antecrochet is promi- 

 nent on the molars, giving a strong curve forward to the 

 median valley. There is a small groove running up the 

 protocone. 



The slender premaxillaries supported the first incisor 

 teeth only, which were found separated from the 

 skull. The nasals, extending out to the ends of the pre- 

 maxillaries, are unusually slender. This is commonly 

 considered a sexual difference, but it is here presumed to 

 be a feature of the earlier stages of the evolution, for no 

 Middle Oligocene forms are known to have the rugose 

 horn supports. 



This specimen is probably one of the progenitors of C. 

 tridactylus which leads ultimately to the parallel-lophed 

 rhinoceroses of the Great Plains, where in each succeeding 

 stage the enamel folding becomes more and more com- 

 plicated. C. tridactylus is longer in the tooth series by 

 the length of JVP ; G. copei is shorter by that amount. 



