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Show 52 MR. O. THOMAS ON THE [Jan. 5, molar teeth very large, hypsodont, and like those of a Rhinoceros 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. N o w this Hetero-hyrax has the essential dental characters of Dendrohyrax 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. O n 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, distinguishes 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 mammae. These three species have a mammary formula of 0 -1 = 2, while certainly in P. syriaca, ruftceps, 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 formula? of" Dendrohyrax grayi " and P. emini cannot be forecasted, and therefore particularly need observation. In any case, however, this 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 difficult 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 (supra, 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 fuiiv c o n. firmed by Woodward, whose discovery of a corresponding lower milk-canine is of much interest. |