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Show 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 examined 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. 1,1.) and in the vestigial condition of the 2nd and 3rd upper incisors and of both canines. The clue to the ultimate suppression of the hinder incisors and the lower canine, and to the vestigial nature of the upper canine and both first premolars in the second dentition, appears to m e 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 premaxillo- 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 mammals 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 |
