1892.'] SPECIES OF THE HYRACOIDEA. 55 



agej while its fusion or non-fusion with the other bones of the skull, 

 although generally constant, is a character rather more variable than 

 has been supposed. 



In a young specimen of one of the species in which it is generally 

 distinct through life, the bone is clearly marked, ordinarily broadly 

 trigonal in shape, its broad posterior end generally embraced by two 

 little processes of the supraoccipital, but these vary very much in their 

 development. At this stage its edges are vertical to the plane of its 

 surface, or if there is any slanting, it is in such a direction that the 

 inner cerebral aspect is rather smaller than the outer. This condi- 

 tion of things remains constant up till somewhere about Stage V., 

 when the ever extending temporal muscles begin to encroach in its 

 vicinity. These muscles seem to induce the development as part of 

 the parietals of a roughened surface-layer of bone, which, with the 

 muscles, gradually creeps onwards over the brain-case, and by de- 

 grees encroaches on and covers up the interparietal bone. The two 

 parieto-interparietal sutures therefore constantly get closer together, 

 the interparietal bone naturally appearing narrower and narrower, 

 and at last the two temporal ridges, which have already met some 

 time before anteriorly, gradually coalesce further and further back, 

 and finally block out all trace of the interparietal bone on the upper 

 surface. Even then, however, for along period the bone may remain 

 uncoalesced, its sutures, in section, describing a curved line following 

 the increase of the parietal bone over it. This gradual narrowing 

 upwards of the interparietal may be seen well in the British Museum 

 skull. No. 69. 10. 24. 41, of P. abyssinica, in which, although the 

 bone itself is broken away, the sutural edge of the parietal clearly 

 exemplifies the steady extension of their upper layers at the expense 

 of the smaller bone between them. 



Now as to the closing of the parieto-interparietal sutures, the 

 early obliteration of which is the main character on which the group 

 " Heferokyrax" rests, some words are necessary, as although 

 really useful in many cases for specific determination, yet the cha- 

 racter is not one that can be used for breaking the family up into 

 smaller groups. In the great majority of the species these sutures 

 are ordinarily persistent and visible, except in so far as they are 

 covered up in the manner above described. On the other hand, in 

 P. brucei they close up so soon that in two specimens as young as 

 Stage III. they have quite disappeared, and in one of Stage II. 

 they are only faintly visible. But two closely allied species, here 

 provisionally admitted as such, but really only doubtfully distinguish- 

 able from P. brucei, have either persistent sutures (P. latastei), 

 or temporary ones, closing up as the animal gets fully adult 

 (P. bocagei), thus proving that the character is at most only of 

 specific importance. 



A second character on which much stress has been laid, and one 

 which has been supposed to be above all suspicion of variability, is 

 the completion or non-completion of the orbit behind by bone. 

 This is always accepted as the essential character of the group "Z>e«- 

 drohyracc," and certainly, in the most typical species, P. dorsalis, 



