54 MR. O. THOMAS ON THE [Jan. 5, 



As to the height of the teeth, by which their hypsodontism or 

 brachyodontism can be gauged, the height of the crown of m^ 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 may 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, arbor ea, 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, 88 ; 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 con- 

 fusion 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 he 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 " Hetero- 

 hyrax" as a subgenus by Lataste practically depended on the age at 

 which it is united to the other bones of the skull. On the examin- 

 ation of a large series of specimens, however, it appears that this bone 

 is by no means really so important as has been supposed for diag- 

 nostic purposes, and that its differences in shape are really due to 



^ Not the extreme antero-extemal cusp, -which has no valley on its outer side ; 

 the cusp measured from is nearly always the highest one of the tooth. 



