52 MR. o. THOMAS ON THE [Jan. 5, 



molar teeth very large, hypsodont, and like those of a Rhiuoeeros in 

 character, while the latter have small, brachyodont, Palseotherium-like 

 teeth ; and these two extremes have been commonly looked upon as 

 the types of distinct genera, respectively Procavia { = Hyrax) and 

 Dendrohyrax. But unfortunately there is really almost a perfect 

 graduation in characters from the one extreme to the other, the chief 

 link being formed by P. brucei, a species which has been made the 

 type of a third genus or subgenus, Heterohyrax. Now this Hetero- 

 hyrax has the essential dental characters of Dendrohyracc combined 

 with the skull of Procavia ; while the one cranial peculiarity supposed 

 to be characteristic of it, the early closure of the interparietal 

 sutures, is not present in P. latastei, a species otherwise in every 

 respect identical with P, brucei. On the other hand, the perfect 

 orbits characteristic of Dendrohyrax occur in a form called "D. grayi" 

 which, except for this one character, does not differ either cranially 

 or externally from P. bocagei, and will perhaps prove to be only a 

 variety of that animal. One single external character, however, dis- 

 tinguishes the three most typical Dendrohyraces, P. dorsalis, arborea, 

 and valida, from all the other species in which the point has been 

 noticed, namely the number of the mammse. These three species 

 have a mammary formula of — 1^2, while certainly in P. syriaca, 

 rujiceps, abyssinica, shoana, welwitschii, and brucei, and therefore, 

 judging from analogy, probably in P. capensis, pallida, bocagei, and 

 latastei, there are 1 — 2 = 6. The mammary formulae of " l)e;ic?ro- 

 hyrax grayi " and P. emini cannot be forecasted, and therefore par- 

 ticularly need observation. In any case, however, tliis character 

 cannot be used as of generic value, for it separates " Dendrohyrax^' 

 equally from both " Heterohyrax " and " Procavia," although the 

 teeth prove that, if anywhere, the division should come between 

 Heterohyrax and Procavia. But even then one would not know 

 into which group to place such an annectant form as P. welwitschii. 

 Balancing, therefore, these considerations for and against the retention 

 of Dendrohyrax and Heterohyrax, I have come to the conclusion 

 that it is better on the whole to recognize only a single genus for the 

 whole of the Hyraces, which will of course bear the name of Procavia. 



Before passing to the actual descriptions of the different species, a 

 few words are necessary as to the cranial, dental, and external 

 characters found in the group, as a proper understanding of these is 

 essential to anyone trying to work out the species of this most 

 diflScult group. 



Firstly, it may be noted that, thanks to the work of Lataste already 

 quoted, and still more to the important paper by Mr. Woodward 

 just read to the Society {supixt, p. 38), the homologies of the 

 teeth are fortunately quite clear throughout. Especially noteworthy 

 is the discovery of rudimentary outer milk-incisors, a discovery 

 which proves the persistent incisors to be really homologous with 

 the first incisors of normal mammals. Lataste's determination of 

 the anterior upper cheek-tooth of the milk-series as mc is fuHy con- 

 firmed by "Woodward, whose discovery of a corresponding lower 

 milk-canine is of much interest. 



