116 Wortman — Studies of JEocene Mammalia in the 



described* as the genus Oxycenodon, from the Uinta. In this 

 group, the species are of uniformly smaller size, one of which 

 is among the smallest of the known Creodonts, and all of 

 them, as far as I am at present aware, are characterized by 

 shallow, remarkably thick jaws, provided with a disproportion- 

 ately large symphysis. A single species of a closely allied 

 form was described by FilhoLfrom the Phosphorites of Quercy 

 in France, under the name of Thereutherium thylacodes. Tbis 

 species is the latest and most specialized of the Limnocyon 

 series, since, according to Filhol's figures, the last superior 

 molar is much reduced, the first premolar has become single- 

 rooted, and the internal cusp of the trigon in the lower molars 

 has disappeared. 



It will be observed, therefore, that we have in this family 

 two distinctive series or lines of descent, one beginning in 

 Oxywna and culminating, according to our present knowledge, 

 in jpatriqfelis, as I have already very fully pointed out,f and 

 a second series beginning in Limnocyon and terminating in 

 Thereutherium. These two series I choose to regard as two 

 distinct divisions or subfamilies, which may be designated, 

 according to the characteristic genera, the Oxyseninse and the 

 Limnocyoninse. 



Owing to our imperfect knowledge of the osteology of the 

 Limnocyoninse, the definition of the Oxygenidse cannot be very 

 fully given at present, but the teeth are sufficiently known 

 to afford very good grounds for distinction from the other 

 families of the Creodonts. I define it upon the dentition as 

 follows : 



Two subequal tuberculo-sectorial lower molars, in which the 

 internal cusp of the trigon and the tubercular heel progres- 

 sively decrease in size or disappear in the later forms ; second 

 inferior molar slightly larger than first ; two superior molars, 

 of which the last is transverse when present, but becomes small 

 or disappears in advanced stages of evolution ; most highly 

 developed sectorial teeth consisting of first molar above and 

 second molar below,. but fourth premolar above and first molar 

 below also sectorial ; two external cusps of first superior molar 

 tending to unite, and internal cusp becoming reduced or disap- 

 pearing in advanced forms, as in Hysenodontidse. 



The two subfamilies may be distinguished by the following 

 characters : 



Lower jaw of considerable vertical depth and not especially 

 thickened from side to side ; symphysis not particularly enlarged ; 

 fibula not articulating with calcaneum, and trochlear surface of 

 astragalus ungrooved, with head very oblique. Oxyaeninse. 



* Bull. Amer. Mus. Nat. Hist., June, 1899, p. 145. 

 f Bull. Amer. Mus. Nat. Hist., 1894, p. 163. 



