Proceedings of the General Meetings for Scientific Business of the Zoological Society of London 1892

Page 82

54 MR. O. THOMAS O N T H E [Jan. 5, As to the height of the teeth, by which their hypsodontism or brachyodontism can be gauged, the height of the crown of nr3 has been taken as the basis. It is measured, in a tooth as unworn as possible, from the top of the main anterior cusp* to the bottom of the valley on the outer side of the cusp ; when there is any trace of a cingulum it is placed at this point, but when there is not, the point at which the valley merges into the smooth basal outer surface of the tooth m a y always be clearly made out. The upper incisors of the Hyraces are of two forms, the one sharply ridged and angular in front, and the other more or less rounded or even flat anteriorly. This difference, though often incidentally-noticed, never seems to have been referred to sex, of which, however, I believe it affords a constant index. The study of so large a series of specimens as the present proves conclusively that all the specimens with ridged incisors are males, and those with them rounded are females. It is of the greatest value to have this ready index to the sex of skulls, as so large a number are either without skins, or if these exist they are unsexable. It should be noticed, however, that in certain species, notably P. dorsalis, arborea, and brucei, the female incisors are also to a certain extent ridged, but to a degree very slight in comparison with those of the male sex. Other differences due to sex seem to be few and unimportant. Even as to size, although male skulls on the whole are rather larger than female ones, yet individual female skulls often exceed the great mass of the males. To take an instance, in P. shoana 5 male skulls give the following lengths-89, 90, 91, 92, 93, and 6 females the following-82, 84, 84. 85, 87, 8 8 ; but one of the types of the species, unquestionably a female, has a length of 96 mm., thus exceeding any of the males as yet recorded. Almost precisely the same thing occurs in the series of P. capensis, where our largest specimen is a female. No general rule therefore can be laid down as to the relative sizes of the two sexes. Throughout the history of the systematic arrangement of the Hyraces the interparietal bone has had a large share in causing con­fusion owing to the alterations that take place in its shape during life not being allowed for or understood. It was early seen that different specimens had very differently shaped interparietals, and in the absence of good series at different ages these differences were naturally supposed to be of specific value. Thus Hemprich and Ehrenberg in 1828 laid primary stress on the shape and form of this bone in separating the four species they recognized; while much later Gray referred a great deal to it, and the retention of " Hetero­hyrax" as a subgenus by Lataste practically depended on the age at which it is united to the other bones of the skull. On the examin­ation of a large series of specimens, however, it appears that this bone is by no means really so important as has been supposed for diag­nostic purposes, and that its differences in shape are really due to 1 Not the extreme antero-external cusp, which has no valley on its outer side • the cusp measured from is nearly always the highest one of the tooth.