1887.] THE PTGMY HIPPOPOTAMUS. 613 



and accordingly all the species of Hippopotamus may be referred 

 to a single genus " ^ 



The new specimen affords an interesting corroboration of this 

 opinion. In the front of the lower jaw are the two usual incisors; but 

 on the right side is an additional smaller tooth placed between the 

 normal incisor (i. 1) and the canine, and which evidently corresponds 

 with the smaller outer incisor of H. amphibius (i. 3, according to 

 Lydekker's determination). This tooth is procumbent (though 

 rather less so than the contiguous incisor). It has a cylindrical root, 

 and (as in the opposing outer incisor of the upper jaw) a portion of 

 the enamel-covered crown remains, the greater part being worn 

 obliquely away. Its diameter is 7 millimetres, that of the first incisor 

 being 12. There is no corresponding tooth on the left side. 



The remaining dentition of the Pygmy species is essentially that of 

 Hippopotamus, although undoubtedly diiferences in detail can be 

 pointed out. The most striking of these are the larger development 

 and greater persistence of the first premolar, the smaller relative size 

 of the fourth upper premolar, and the greater simplicity of the form 

 of the crowns of the true molars. Whether these characters are of 

 sufficient importance for generic distinction is a point to be decided 

 according to the view taken of the advisabiUty or otherwise of 

 multiplying such distinctions. 



With regard to the cranial differences, so strongly insisted upon 

 by Leidy, Milne-Edwards, and Gratiolet, striking as they are on 

 superficial observation, they all depend upon one circumstance, 

 the greater relative size of the brain-cavity and capsules of the 

 sense-organs (orbits and auditory bullae) in H. liberiensis, contrasted 

 with the huge development of the masticating organs, and ridges 

 for the attachment of muscles to move the jaws, in H. amphibius. 

 Apart from this the crania are essentially similar, even the remark- 

 able thin-walled capsule formed by the lachrymal bone in the floor 

 of the orbit, well known in the common species, is present, though 

 on a smaller scale, in the Libeiian animal. Now it is rather 

 remarkable that these differential characters have been pointed out 

 with °-reat emphasis by the three eminent anatomists mentioned 

 above, without any indication of the circumstances that they are 

 just those characters by which, in any natural group, the small 

 members differ from the large, and just those in which in any species 

 the young differ from the adult. The universal law of the arrest of 

 growth of the nervous system and sense-organs in the large members 

 of homoo-eneous groups fully accounts for all the differences of the two 

 skulls which have been pointed out with such minuteness. Exactly 

 similar differences are found between the Tiger and the smaller species 

 of Felis, the Gorilla and Baboons and the smaller aUied Apes, the 

 large and small members of the genus Otaria, and in fact wherever 

 there is great diversity in size in closely related forms and even in 

 individuals of the same species. A pygmy Hippopotamus which 

 should present all the exact proportions of the large form as regards 

 these parts of its organization, would be as great an anomaly as a dwarf 

 1 Palseontologia Indica, ser, 10, vol. iii. p. 47. 



