222 



THE AGE OF MAMMALS 



Among carnivores the archaic hyanodonts also appear for the last 

 time, and are represented by rather highly varied forms, animals dis- 

 similar in size, speed, and in the proportions of the skull, ranging from 

 the diminutive H. mustelinus to the powerful H. horridus (Fig. 81), 

 and including also species {H. leptocephalus) which exhibit in the closure 

 of the posterior palate a backward extension of the respiratory tract 

 which has been regarded (Scott) as evidence of aquatic adaptation,^ but 

 may be correlated with the extreme posterior position of the cutting or 

 sectorial molars. At the same time the machserodont cats specialized into 

 the fleet and slender-limbed, swift-moving Dinictis and the heavy-limbed 

 H oplophoneus (Fig. 87), which is transitional to the Eusmilus of the Upper 



Fig. 109. 



-Skeleton of the Uligocene wolf, DapJuvnus. In the Carnegie Mu.scum, Pitt.sbiirg. 

 After Peterson. 



Oligocene. The canids ' also vary widely from the small civet-like Cyno- 

 didis to powerful forms such as Daphoenus nebrascensis, which equaled 

 the wolf (Canis lupus) in size. 



The Herbivora which formed the prey of these carnivores are included 

 in six families of artiodactyls and six families of perissodactyls, these two 

 orders at the time being nearly balanced both in numbers and differentia- 

 tion. Of the artiodactyls the camelids (Poehrotherium) , which are now of 

 slender form, begin to take a prominent part in the Plains fauna. The 

 hypertragulids, or primitive ruminants and deer, are still diminutive and 

 hornless forms. The oreodonts are of intermediate size and now more 

 sharply differentiated into three phyla, (1) the small brachycephalic Lep- 

 tauchenia being added to (2) the typical cropping or grazing oreodonts and 

 to (3) the forest-living Agriochoerus. Diminutive also are the leptochoerids. 

 Of intermediate size are the anthracotheres, which include both the An- 



' Scott, W. B., and Osborn, H. F., Preliminary Account of the Fossil Mammals from the 

 White River Formation, contained in the Mus. Comp. Zool. Bull. Mas. Comp. Zool., Harvard 

 Coll., Vol. XIII, 1887, pp. 152 fol. 



^ Hatcher, J. B., Oligocene CanidiE. Mem. Carnegie Mus., Vol. I, Sept., 1902. 



