400 MAMMALIA. 



postorbital constriction of the brain-case. The neck is comparatively long, 

 and the tail excessively elongated. The limbs are rather stout, and the 

 digits must have borne completely retractile claws; but the feet are 

 comparatively short and weak, as in the Creodonta. The humerus 

 exhibits an entepicondylar foramen ; the femur bears a distinct third 

 trochanter. The scaphoid and lunar bones of the carpus are united, but 

 their original line of demarcation can still be distinguished. The most 

 completely known species is Hoplophoneus primcevus, an animal somewhat 

 less than a metre in length (exclusive of the tail), from the Lower Miocene 

 (White River Formation) of Nebraska, Colorado, and Dakota. 



FIG. 224. 



Nimravus gomphodus ; skull, mandible, and neck, left lateral aspect, two-fifths 

 nat. size. Middle Miocene (John Day Formation) ; Oregon. 3, 4, third and 

 fourth premolars ; 1, 2, first and second molars. (After Cope.) 



Nimravus (fig. 224). A genus with a less specialized dentition than 

 that of Hoplophoneus, the upper canine being less disproportionately large 

 and the hinder teeth less reduced in number. There are three upper 

 premolars, and one transversely-elongated molar ; while in the mandible 

 there are two premolars in front of the sectorial molar, with a diminutive 

 tubercular m. 2 behind it. The femur has lost its third trochanter. The 

 typical species is Nimravus gomphodus (fig. 224) from the Middle Miocene 

 (John Day Formation) of Oregon. 



Machaerodus (fig. 225). This genus, in its wide sense, includes the 

 largest and latest species of the Nimravidse, commonly known as " sabre- 

 toothed tigers," with enormous laterally compressed upper canines, the 



