| OCR Text |
Show 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-development 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 practicable for all species, owing to the fact, observed by Lataste, that the time of the fall of the milk-premolars as compared with the development 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 m a y 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 1 up ; p^ below level of bone. IV. m^ just appearing or partly up. V. nr* nearly or quite up ; m^ below level of bone. VI. Tip of m 3 appearing. VII. m3 partly or nearly up, but still unworn. VIII. m_3 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 n£ at its broadest point, this tooth being present and available in specimens at all ages from Stage III. upwards. |
