NO. I THE WHITE RHINOCEROS — HELLER 23. 



classed as immature, but only those, however, which have all their 

 milk molars in use are actually immature. This state of affairs is 

 brought about in Ceratotheriitm, owing to the retention of part of its 

 milk dentition long after it has begun to breed and has virtually 

 become an adult animal. Immaturity, as here used, simply means milk 

 dentition, either complete or partial, without any reference to the 

 breeding age or size of the specimen. In the great majority of hoofed 

 animals the milk dentition corresponds with actual immaturity, and 

 the term is employed here on the same dental basis to conform to such 

 usage. The adults are those individuals which have shed all their 

 milk molars, and have their permanent premolars in use, but having 

 the last molar just erupting. They are animals which have attained 

 middle life. Those showing wear on the last molar are classed as old. 



The variation due to age concerns itself largely with the occipital 

 prolongation, the union of the premaxillse, the deflection or dip and 

 the rugosity of the nasals at the expanded tip, and the development of 

 the processes springing from the floor of the cranium. All of these 

 characters are intensified in age together with a general development 

 in massiveness and accentuation of the sexual characters found in the 

 skull. In the nursing young the skull shows no occipital prolongation 

 beyond the condyles, being at this age practically like Diceros in this 

 regard. They are quite similar in general shape to the skulls of 

 Diceros of the same age, but are distinctly narrower and dolicho- 

 cephalic with less concavity to the dorsal outline. Soon after leaving 

 this age there is a gradual prolongation which continues well on to 

 maturitv before reaching its maximum development where the 

 extreme dolichocephalic condition is reached. 



The premaxillje consist of two very small bones, an inch or two 

 long, projecting beyond the heavy maxillary bones, but not uniting 

 with one another for some time after the animal has become adult. 

 They are widely separated at birth and remain thus until most of the 

 milk premolars have disappeared. They usually unite along only a 

 part of their width. In only one skull are they united broadly to form 

 a solid beak-like symphysis. 



The nasal bones near their tips are smooth and raised into a promi- 

 nent boss from which there is a sharp descent to their extreme tip. 

 As age proceeds this boss expands in width and becomes lower, the 

 angle made by the anterior and posterior surfaces of the boss being 

 much greater in old age. Accompanying this greater development 

 of the tips of the nasals is a marked increase in the rugosity which 

 is needed for the support of the enlarged front nasal horn. The age 



