218 LEIDY'S OSTEOLOGY OF THE HEAD OF HIPPOPOTAMUS. 



and very little outward in the former, contrasted with the latter. In Choeropsis the 

 inner side of the orifices of the canine alveoli is on a straight line forward from the 

 outside of the molar alveoli, while in Hippopotamus it is at least several inches 

 distant from the same line. 



Dentition. — The formula of the permanent dentition of Choeropsis Liberiensis is, 



2—2 1—1 4—4 3—3 



i, c. p. lu. ni. = 38. 



1—1 1—1 4—4 3—3 



De Blainville, in the memoir " sur les Hippopotames," in his magnificent 

 Osteographie, p. 28, in regard to the incisor dentition, says, "dans le jeune age le 

 nombre des incisives doive etre et sont reelemeut f ; mais le grand ecartement des os 

 incisives et des denies qu' ils portent par suite du developpementde la premiere paire 

 d'incisives inferieures, aussi bien que de celui des canines, a sans doute entraine la 

 perte de la premiere incisive superieure, et determine I'obliteration de la troisieme 

 d'en bas." And a few pages further (p. 32) he observes, " Dans la tres jeune age, il 

 est certain qu' il y a trois paires d' incisives en haut com me en bas, et que ces 

 incisives sont beaucoup plus subegales, dont une paire excessivement petite n' est 

 pas remplacee, la premiere superieurement, la troiseme inferieurement." And, in a 

 note, he adds, " je n' ai reellement vu que celle d 'en bas, formant une petite callote 

 convexe en dedans et en dehors de la seconde ; mais je suppose, par analogic, que la 

 premiere d'en haut etait gingevale et n'est pas restee dans la preparation." 



In an adult specimen of a skull of Choeropsis, described by Dr. Morton, and in a 

 second specimen of a head, in connection with the entire skeleton lately received 

 from Africa by the Academy, in addition to the two upper pairs of functionally 

 developed incisors, in front of those lateral, there exists an elongate conical cavity, of 

 about half an inch in length, communicating with the alveoli of the latter, and 

 containing an ossified rudimentary incisor, of the same size and form of its cavity, 

 worn away at the lower part, where it comes in contact with the contiguous func- 

 tional incisor. In a specimen of an old adult of Choeropsis, the rudimentary incisors 

 have disappeared, and their cavities are obliterated. 



Thus has been established, without doubt, that the normal incisive formula of the 

 Hippopotamidse is : 3 — 3 



3—3 

 as in the ordinary pachydermata. Functionally developed in the extinct Hexapro- 

 todon, they are reduced to 2 — 2 



2 — 2 

 in the Hippopotamus (Tetraprotodon of Falconer and Cautley,) and in Choeropsis un- 

 dergo the excessive reduction of 2 — 2 



1 — 1 



