1887.] 



385 



[Cope. 



y?. Superior canines decurved. 

 Superior canines small Hyotherium Von Myer. 



/?/?. Superior canines recurved. 

 Molars with four much plicate tubercles on each. .HippoJiyus Cautl. Falc. 

 Molars with numerous irregular accessory lobes ; premolars |. .Bus Linn. 

 No accessory lobes ; premolars f Bahirussa Cuv. 



III. Metapodials distinct ; superior incisors reduced in number ; mo- 



lars reduced in number, and the valleys filled with cement 

 (Phacochoerime). 

 Superior incisors one ; premolars none ; molars |, with numerous tuber- 

 cles ; superior canines recuived Phaeochc&rus Cuv. 



IV. Metapodials distinct, distally keeled behind only ; inferior in- 



cisors straight, subcylindric (Hippopotaminse). 



Six lower incisors ; orbit closed Uexaprotodoii Caut. Falc. 



Four lower incisors ; orbit closed Hippopotamus Linn. 



Two lower incisors ; orbit not closed Chceropsis Leidy. 



The absence of intermediate types renders the determination of the 

 phylogeny of the genera as yet impracticable. The main features may 

 however be foreshadowed. The most generalized form is Thinohyus, 

 since its dentition is in all respects the most simple, while it preserves the 

 full number of teeth. It may readily have given origin to the Dicotyline 

 line on the one side, and Sus and its immediate allies on the other. Babi- 

 russa is another derivative from the same center. Phacochcerus may have 

 come from some ally of Sus, since it carries to a great extreme the pecu- 

 liarities of the latter genus. The ancestry of Hippopotamus is less easily 

 determined. Its imperfect distal metapodial keels, which only exist on 

 the posterior face of the condyle, bespeak for it an ancient ancestor. Its 

 molar type is merely a complication of the quadritubercular, while the 

 characters of its canines are an exaggeration of those of the primitive 

 forms already mentioned. Several other genera, as Dicotyles and Sus, 

 display the decumbent incisors which prepare the way for the remarkable 

 straight digging incisors of Hippopotamus. The genus Chceropsis eases 

 the passage backwards. These relations may be expressed as follows : 

 Chceropsis Phacochcerus Platygonus 



Hippopotamus 



I 

 Hexaprotodon 



Dicotyles 



Babirussa Sus 



Chaenohyus 



Thinohyus 



PKOC. AMER. PIIILOS. SOC. XXIV. 12(3. 2w. PRINTED DEC. 3, 1887. 



