382 MAMMALIA. 



acromial process, and the humerus is pierced by an entepicondylar foramen ; 

 the radius is somewhat more slender than the ulna, which bears a very 

 large olecranon process ; the manus is proved to have been pentadactyl, 

 with slightly cleft ungual phalanges. The fibula is nearly as robust as 

 the tibia, and the upper articular face of the astragalus is scarcely sinuous. 

 The typical and best known species is Arctocyon primcevus, from the basal 

 Eocene deposits in the neighbourhood of Rheims, France, with the skull 

 about O21 m. in length. Part of a nearly similar dentition from the 

 Lower Eocene (Wasatch Formation) of Wyoming, U. S. A., bears the 

 generic name of Anacodon. 



Mesonyx. This genus is known by at least one complete skeleton, 

 and by numerous other good specimens. The skull is shaped much like 

 that of the existing thylacine of Tasmania, but the brain-case is relatively 

 smaller and the sagittal crest more prominent. The dental formula seems 



, , i. 3, c. 1, pm. 4, in. 2 , 



to be . TT -r-^- , ; but as the successional teeth are unknown, it 



i. 2, c. 1, pm. 4, rn. 3 



is not quite certain whether the lost tooth in the upper jaw is pm. 1 or 

 m. 3. The three hindermost upper teeth are tritubercular, approximately 

 similar in size and shape ; the three teeth between these and the canine 

 are laterally-compressed cones, with a more or less distinct basal denticle 

 in front and behind. The canines in both jaws are stout and somewhat 

 enlarged. The lower pm. 1 is minute, but the others closely resemble the 

 opposing teeth of the upper jaw, and are followed by three nearly similar 

 but stouter molars. The axis vertebra bears a very large neural spine. 

 There are 14 thoracic or dorsal vertebrae, of which those anteriorly are 

 comparatively small and slightly opisthocoelous, with much-elevated neural 

 spines. These are followed.by 6 lumbar vertebrae, which, like the posterior 

 thoracics, bear metapophyses and are very firmly united. The tail is 

 remarkably elongated. The fore limbs are somewhat shorter than the 

 hinder pair, and the scapura'*ftrsh8;ped much like that of a Hyaena. The 

 humerus is slender and destitute of an entepicondylar foramen ; the 

 carpus includes a well-developed centrale, and all the bones remain 

 separate ; the manus is tetradactyl, with comparatively broad ungual 

 phalanges which are deeply cleft, and digit no. I is represented only by 

 the proximal end of its metacarpal. The pelvis most resembles that of a 

 bear. The femur bears the third trochanter as usual ; the fibula is 

 complete, though -Kejatively slender ; the astragalus is slightly grooved on 

 its upper articular face, while its lower articulation extends well upon the 

 cuboid ; and the pes is tetradactyl, only differing from the manus in the 

 comparative slenderness of the metatarsals, which are nearly equal in 

 length. Mesonyx obtusidens, with the skull O25 m. in length, would be 

 about as large as a smalT brown bear, and is known by a complete skeleton 

 from the Middle Eocene (Bridger Formation) of Wyoming. Other species 

 are represented in the Bridger and Uinta Formations of Wyoming and 

 New Mexico ; and there is also some evidence of the same genus in 



