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.S the other three having closed orbits. On the other hand, 

 the type of Dendrohyrax graiji has them completely closed, as has 

 also a second specimen from the same region ; but I am nevertheless 

 most strongly inclined to consider these two specimens as only re- 

 presenting an individual variation of P. hocagei '. 



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 Socage 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 (pi) is worthy of 

 some notice. In the large-toothed, hypsodont species, such as P. 

 capensis, ahyssinica, &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 elon- 

 gated, has two distinct roots, and is practically persistent throughout 

 life. These differences areclearly 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 (mp^) 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, brucei, and abyssmica) 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 : — 



* On one side ; the other is broken. 



* Bee below p. 72. 



