28 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 6l 



of .the metalophid joins the posterior end of the protolophid enclosing 

 another fossette, which condition lasts until the tooth is greatly worn. 



The permanent premolars and molars differ only in size from the 

 milk molars. They are much larger, but show no more complexity 

 of structure with the exception of the greater development of the 

 cement layer. In the permanent teeth this layer usually fills the 

 cavities of the fossettes. In old age, when the last molar shows wear, 

 the other permanent teeth usually show a very worn condition with a 

 single central island of enamel, the post-fossette. 



There is remarkably little individual variation in the folds or 

 crests of the teeth. Most of the differences which occur between the 

 corresponding teeth of the two sides are due to differences of wear 

 or age. One specimen, number 164594, shows the crotchet of the left 

 fourth upper premolar cut off and isolated as a fossette, but this con- 

 dition is temporary, a little more wear only being required to unite 

 the crotchet as normally. Both the second upper molars of this same 

 specimen show the metaloph isolated as a fossette by a shallow valley 

 at its junction with the ectoloph. As in the case of the premolar, a 

 little more wear would reduce this peculiarity to a normal condition. 



There is no evidence of either incisors or canines in the youngest 

 skull, number 164588, that of a half-grown calf. Skulls of the same 

 age as the latter of Diceros show upper incisors, but of a very rudi- 

 mentary character. In the mandible these young skulls show alveoli of 

 incisors which had been recently absorbed. Ceratotheriuni; it would 

 appear, is a more specialized genus than any of the other living rhi- 

 noceroses as regards its teeth. 



EXPLANATION OF TABLE OF SKULL MEASUREMENTS 



All of the skulls listed, with the exception of the South African Museum 

 specimen, have been examined and measured by the writer. 



1. A. M. — American Museum of N. Y. Skull with horns still attached, but 



lacking a mandible, and with the zygomatic arches and con- 

 dylar region cut away. Collected in South Africa by a mis- 

 sionary in the early part of the last century. Specimen now 

 in the Department of Paleontology. Figured in plates. 



2. B. M. — British Museum. Skull from South Africa belonging to the 



mounted skeleton in the Department of Paleontology in the 

 British Museum. Figured in plates. 



3. H. ,M. — 'Hamburg Museum. Specimen from South Africa lacking a defi- 



nite locality. Associated with a much older mandible than 

 its own tooth row would indicate belonged to it. 



4. C. M. — Congo Museum. Lado Enclave specimen. Now a mounted speci- 



men in the Congo Museum, Brussels. Shot by Major Powell- 

 Cotton. 



