1892.] SPECIES OF THE HYRACOIDEA. 53 



Owing to the comparative lateness in life at which apparently the 

 Hyraces become fully adult, and the consequent frequency with which 

 more or less immature specimens have to be dealt with, special 

 attention has to be paid to the age of every specimen described. 

 For purposes of comparison therefore the period of tooth-develop- 

 ment has been divided into eight stages, mostly determinable by the 

 relative development of a single tooth, and thus by the comparison 

 of specimens of similar ages the true inherent differences in size 

 between different forms become easily definable. A single tooth only 

 is taken as the main determinant of each of the stages, no account 

 of the general state of the dentition at any given stage being practi- 

 cable for all species, owing to the fact, observed by Lataste, that the 

 time of the fall of the milk-premolars as compared with the develop- 

 ment of the permanent molars varies in different species. 



The following are the stages which I have found divide the 

 specimens most conveniently into groups of individuals of similar 

 age. The actual age, in time, at which in the different species 

 these stages are attained may perhaps be found out later at a more 

 advanced period of knowledge : — 



Stage I. Before the milk-dentition is fully in place. 



II. Milk- dentition all up and in use. m^ not visible. 

 III. m^ up ; ^ below level of bone. 

 IV. m^ just appearing or partly up. 

 V. m^ nearly or quite up ; m^^ below level of bone. 

 VI. Tip of ^ appearing. 

 VII. ^ partly or nearly up, but still unworn. 

 VIII. mf up and in use. 



No doubt Hyraces are practically adult, and are probably breeding, 

 some time before Stage VIII. is attained, just as in the Kangaroos 

 and other animals in which there is a horizontal succession of the 

 teeth owing to the movement forward of the tooth-row, and the 

 consequent replacement of the crushed and worn-down anterior 

 teeth by the newly formed posterior ones. Of course the process is 

 not nearly so highly developed as it is in the Kangaroos, Manatees, and 

 others ; but there is evidently a commencement of this remarkable 

 provision for the replacement of the worn-out teeth in the Hyracoidea, 

 especially in the hypsodont species, such as P. capensis, abyssinica, and 

 their allies. 



Thanks to this process, the actual size of the teeth, however 

 valuable for the discrimination of the species, cannot be defined 

 satisfactorily by a simple antero-posterior measurement of the tooth- 

 row or any part of it, for the larger posterior teeth as they push 

 forwards gradually crush together the whole of the teeth and make 

 their combined length less and less as time goes on. 



To gain an idea of the actual size of the teeth, it has therefore 

 been found best to take the exact horizontal breadth of ^ at its 

 broadest point, this tooth being present and available in specimens 

 at all ages from Stage III. upwards. 



