186 PROCEEDINGS OF THE ACADEMY OF [1893. 



amphibius. 8 Thus, for example, Flower, 9 a very high authority, 

 does not consider the difference in the shape of the cranium and in 

 the number of the incisor teeth in the lower jaw as warranting the 

 establishment of the genus Choeropsis. The difference presented by 

 the crania in the two kinds of hippopotamus, Flower regards as 

 similar to those " between the Tiger and the smaller species of 

 Felis, the Gorilla and Baboons and the smaller allied apes." In 

 the judgment of the author, however, it may be at least questioned 

 whether the differences existing between the smaller species of Felis 

 do not justify separating them into distinct genera. On the 

 other hand, although the Gorilla has descended in all probability 

 from some Baboon-like form, zoologists do not as yet recognize these 

 two apes as species of the same genus. The fact that Hippopotamus 

 amphibius syn. Tetraprotodon has, according to Gaudry, 10 exhibited 

 in one instance unilateral hexaprotodontism and Choerop>sis, accord- 

 ing to Flower, 11 in one instance unilateral tetraprotodontism would 

 influence but few palaeontologists in regarding, like Lydekker, 12 

 Hexaprotodon, Tetraprotodon and Choeropsis as merely species of 

 one genus Hippopotamus. Hexaprotodon and Tetraprotodon, with 

 the incisor formula f-f and f-| respectively, are still consid- 

 ered either as sub-genera, as they were originally by Falconer and 

 Cautley, 13 or as genera, as by the greatest of British palaeontologists, 

 the late Sir Richard Owen. 14 The latter view being accepted by 

 the author, Choeropsis, with the incisor formula f-f, and differing 

 in other respects far more from the living hippopotamus (Tetrapro- 

 todon) than the latter does from the extinct one (Hexaprotodon), 

 should certainly be regarded as a genus distinct from Hippopotamus. 

 It appears to us that too much importance has been attached by 

 Lydekker and Flower to the presence of an extra incisor tooth in 

 the lower jaw of Hippopotamus amphibius and Choeropsis respect- 

 ively, especially as it has only been noticed once in either case. We 

 would rather regard the presence of such an incisior tooth as an 

 individual peculiarity and as an instance of redundancy than of 

 reversion. In view of what has already been urged by Leidy, 



8 Carus, Zoologie, 1868, p. 145. 



9 Pro. Zool. Soc. London, 1887, p. 612. 



10 Bull. Soc. Geologique, Ser. 3, Vol. 4, p. 504. 



11 Op. cit. 



12 Memoirs of the Geological Survey of India, 1884-1886. Vol. 3, p. 47. 



13 Falconer, Palseontological Memoirs, Vol. 1, 1868, p. 140. 

 " Odontography, 1840, p. 566. 



