| OCR Text |
Show 1892.] SPECIES OF THE HYRACOIDEA. 55 age, 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 m u c h 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 condition 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 degrees 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 m a y 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. N o w as to the closing of the parieto-interparietal sutures, the early obliteration of which is the main character on which the group "Heterohyrax" rests, some words are necessary, as although really useful in many cases for specific determination, yet the character 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 distinguishable 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 "Dendrohyrax," and certainly, in the most typical species, P. dorsalis, |
