OCR Text |
Show 56 MR. O. THOMAS ON THE [Jan. 5, the orbit is closed, even in the youngest specimens available, down to Stage I. In P. arborea, however, the South-African form, one of our four specimens, already fully adult (Stage VIII.), has the postorbital processes of the frontal and malar separated by at least 2 mm.1, the other three having closed orbits. On the other hand, the type of Dendrohyrax grayi has them completely closed, as has also a second specimen from the same region ; but I a m nevertheless most strongly inclined to consider these two specimens as only representing an individual variation of P. bocagei 2. As it appears therefore from both these characters that the Angolan Hyrax is the one which presents the greatest difficulty, I would strongly impress on collectors having the opportunity the great desirability of obtaining more specimens from that country. In this connection I must again thank Prof. Barboza du Bocage for the loan of the valuable Angolan specimens preserved in the Lisbon Museum, a loan which has been of the very greatest service to me. The development of the anterior lower premolar (p"1) is worthy of some notice. In the large-toothed, hypsodont species, such as P. capensis, abyssinica, & c , it is a simple slender tooth, with only one root, and is pushed out by the teeth behind it at a very early age, so that it is quite unusual to find it present in fully adult animals. On the other hand, in the small-toothed brachyodont species it is elongated, has two distinct roots, and is practically persistent throughout life. These differences are clearly correlated with the amount of the wear and tear of the teeth and their movement forwards in the jaw, characters at their maximum in the hypsodont and their minimum in the brachyodont species. The predecessor of this tooth (mp1) is always long and double-rooted, showing clearly which of the two adult forms is the primitive one of the group. The number of the ribs has also been used as a distinguishing character of the genera and subgenera, but, so far as I have seen, all the species examined (including such widely separated species as P. dorsalis, bructi, and abyssinica) have 21 (rarely 20), while P. capensis alone has 22. I have, however, thought it worth while to record the numbers in the specific descriptions wherever I have direct knowledge of them. Of the external characters by which the different species may be determined, the most important are the coloration, shape, and size of the dorsal spot, a patch of hairs growing on and around the dorsal gland, and almost invariably of a colour markedly contrasting with that of the body in general. The following are the chief variations in the dorsal spot met with in the different species :- 1 On one side ; the other is broken. 2 See below p. 72. |