SUBORDER A 



CREODONTIA 



51 



The Mesonychidae usually attain the size of bears, and their extremities 

 resemble those of the Canidae, hence they were fleet and strong runners. 

 The carpals and tarsals are compactly set, astragalus Math deeply 

 grooved trochlea, the first digit has become atrophied, and the metapodials 

 are parallel and digitigrade. The upper molars are distinguished by 

 having three cusps but lacking a metastyle, and the lower by the reduction 

 of the metaconid and the simple form of the talonid, thus being secondarily 

 triconodont. Upper and lower molars never form a shearing surface. 

 Distributed in the Lower Eocene of North America, France, and Belgium. 



Subfamily 1. Triisodontinae Matthew. 



P f . M % ; or P ^, M f . Trigonid of lower molars with inner cusps, 

 talonid broad, concave. Outer cusp of superior molars isolated. 



Triisodon Cope. Eoconodon Matthew and Granger. Puerco beds. 

 SarcothraiLstes, Goniacodon, Microclaenodon Cope. Torrejon beds. 



Subfamily 2. Mesonychinae Matthew. 



Lower molars with talonid trenchant. Outer cusps of upper molars fused. 



Dissacus Cope. M -|. Anterior lower molars with strong metaconid. 

 Humerus with entepicondylar foramen ; extremities pentadactyl. Wasatch 



Fig. rr. 



Manus of Mesonyx (A) and of Hyaenodon (B) about 2/3. sc, Scaphoid ; I, lunar ; c, cuneiform ; ce, centrale ; 

 tz, trapezium; M, trapezoid ; m, magnum; u, unciforme ; I.-V., first to fifth metacarpals. C, The last two 

 phalanges of Hyaenodon. 



and Torrejon beds of New Mexico ; Wasatch and Wind River beds of 

 Wyoming. Basal Eocene near Rheims, France. 



Hyaenodictis Lemoine. Basal Eocene of Cernays near Rheims, and 

 Erquelinnes, Belgium. 



