LEIDY'S OSTEOLOGY OF THE HEAD OF HIPPOPOTAMUS. 219 



Dr. Falconer, in a letter to Dr. Morton,* makes use of the following singular 

 language : "Hippopotamus comprises two subgenera, Hexaprotodon, with six incisors 

 above and below ; and Tetraprotodon, the species with reduced incisors, viz., four 

 above and below. Your species," referring to the Choeropsis Liberiensis, 

 "although it has but two in the lower jaw, belongs to the latter subgenus. The 

 excessive reduction is probably only an individual case of variety, but if proved 

 to be constant, the position of the species in the genus will not be altered." 



It would indeed appear singular if the functional development of the normal 

 number of incisors should be thought sufficiently worthy to consider Hippopotamus 

 Sivalensis a distinct subgenus from the existing Hippopotamus, and a reduction from 

 the number as existing in the latter, should not alone entitle Choeropsis to the same 

 distinction. But, in addition to the reduced number of incisors in the latter, the 

 important differences, which have been already designated, between the head of 

 Choeropsis and that of Hippopotamus, in addition to others which will be pointed out, 

 more especially in the constitution of the upper incisors, quite as remarkable as the 

 minimum number below, induce me to believe, that if the genus Hexaprotodon be 

 adopted as distinct from Hippopotamus, Choeropsis must be admitted without 

 hesitation. 



The two inferior incisors of Choeropsis are nearly straight, being bent in their 

 length slightly downwards, and project obliquely forward and upward, as in Hippo- 

 potamus, their size corresponding to those lateral of the latter. As in this, when 

 young, their protruded portion is capped with enamel, which being worn off leaves 

 them as two rods of dentine, of persistent growth. 



In neither of the specimens, of the lower jaw of Choeropsis which we possess, have 

 I observed any rudiments of the deficient incisors, but it is not improbable they may 

 be concealed within the jaw. 



In the upper jaw of Choeropsis the incisors all curve in the same direction forwards 

 and downwards; the protruding portion being directed vertically downward. In the 

 young animal, enamel envelopes the protruding portion of these teeth, but no bands 

 continue down their whole length, so that in the adult they are persistently growing 

 rods of dentine, entirely devoid of enamel, as in the lower incisors in this genus and 

 Hippopotamus, and as is also the case with the supero-lateral incisor of the male 

 specimen of a head, which has been indicated, of Hippopotamus Capensis. 



The upper incisors of Choeropsis are of course opposed to the lesser number below. 

 The abraded surface of the superior mid incisors is a horizontal, oval convexity ; that' 

 of the lateral incisor, an oblique, oval disk, directed inwardly. The distal extremity 

 of the inferior incisors presents an upper, horizontal surface, oblique in its relation to 

 the length of the tooth, and divided into two irregular abraded concavities, separated 



* Journ. Acad. Nat. Sci., of Philada., new series, toI. 1, p. 236. 



