1892.] MILK-DENTITION OF PROCAVIA CAPENSIS. 43 



below, are much reduced (though well developed as milk-teeth), 

 often bearing but a single fang, and are early shed. 



It appears to be generally the rule amongst herbivorous animals 

 that there is a suppression of the anterior teeth of the maxillary 

 series, often accompanied by a reduction in number of the incisors 

 and an increase in size of the remaining teeth. From this it would 

 seem probable that Hyrax, in which the food and habits are very 

 similar to those of the Rodentia and herbivorous mammals, would, 

 find it more serviceable to possess one or two pairs of large cutting 

 incisors than three smaller ones ; and the very early development and 

 large size of the anterior incisors and of the large grinding premolars 

 becomes readily intelligible upon this hypothesis. 



If the jaws of the foetus be exan)ined in relation to the entire 

 milk-dentition and to the rest of the skull, they will be seen to be 

 relatively very small. Comparison of a series of skulls shows that 

 the elongation of the jaws is attained very late, and that above it 

 mainly involves the premaxillary and anterior maxillary regions. 

 The conditions are such that in the young animal the jaws are 

 uncomfortably crowded with teeth; this is especially noticeable in 

 regard to the position of the first premolars (fig. 1, i.) and in the ves- 

 tigial condition of the 2nd and 3rd upper incisors and of both canines. 



The clue to tlie ultimate suppression of the hinder incisors and 

 the lovier canine, and to the vestigial nature of the upper canine and 

 both first premolars in the second dentition, appears to me to lie 

 in the consideration of the above facts. 



The large size and early development of some of the teeth render 

 it obvious that these would take up more than their proper share of 

 space and nourishment in the already overcrowded jaw, and we 

 accordingly find that some of the teeth, which were of least 

 functional importance, become either stunted or entirely crowded 

 out of the jaw before it elongates. 



The premolars of the foetus (fig. 1) extend forwards to the pre- 

 maxillo-maxillary suture ; and comparison with the adult shows that 

 the diastemata possessed by the animal are more nearly the result 

 of a secondary elongation of the jaws themselves than of the mere 

 suppression of certain teeth. 



III. General Considerations. 



It will be seen from the foregoing description that Hyrax possesses 

 several teeth which are only represented in one dentition. The 

 question then arises whether these are to be considered as belonging 

 to the 1st or to the 2nd series, and the answer to it involves a brief 

 consideration of the relations existing between the two dentitions in 

 the class Mammalia. 



Prof. Flower (6, 7, 8, 9), in his various contributions to the study 

 of Mammalian odontology, has all along sought to show that mam- 

 mals were primitively monophyodont, and that the original single set 

 of teeth is represented in the permanent (successional) teeth of the 

 Diphyodonts, the possession of a milk or first dentition being a 



