Marsh Collection, Peabody Museum. 41 



to the plantar surfaces ; the phalanges are long and slender, 

 and the bony claws are rather narrow and deeply fissured. 

 The measurements are as follows : 



Length of tibia _ . 222'5 mm 



Length of foot _ . 236- 



Length of tarsus 89* 



Length of third metatarsal 87' 



Length of fifth metatarsal ._ 76* 



Length of second metatarsal _ _ _ 19' 



Length of first phalanx, digit II 33* 



Length of first phalanx, digit III _ 32* 



Length of first phalanx, digit V 27* 



Length of second phalanx, digit III 17' 



Length of ungual phalanx, digit III 18* 



Discussion. — Attention has already been drawn to the fact 

 that the phylogenetic history of this subfamily is unusually 

 complete, from the time when we first meet with it in the 

 Torrejon Beds, up to the time of its final extinction at the 

 close of the Eocene. Although the representatives of the 

 last, or Uinta, stage, are not fully known, there are very good 

 reasons for the belief that Mesonyx represents essentially as 

 high a degree of perfection as the group attained. The 

 amount of change in its osteological structure does not, at 

 first sight, appear to be great, yet the gradual modification of 

 the limbs and feet from the short, broad, pentadactyle, planti- 

 grade condition of Dissacus, to the elongated, compressed, 

 tetradactyle, highly digitigrade feet of Mesonyx, almost if not 

 quite equal to the Greyhound in point of specialization of 

 these characters, is an accomplishment which the modern dogs, 

 with the advantage of a much longer existence, have only 

 comparatively recently achieved. 



Origin of the Tritubercular Molar. 



The modification of the skull and teeth, while not of so 

 pronounced a character as that of the limbs, is nevertheless 

 very apparent. The molars of the inferior series have grad- 

 ually lost the internal cusp of the trigon, and the pattern of 

 the crown has, in consequence, assumed a simpler orpremolari- 

 f orm structure. It is manifestly improper to say, however, that 

 they underwent any degenerate changes, since at no point in the 

 history of the phylum do they show any disposition or exhibit any 

 tendency to decrease in size or become less functional than in the 

 earliest members. The loss of this cusp is paralleled in many 

 groups of the Carnivora, but in all such cases thus far known 

 its disappearance has been accompanied by the development 

 of a more perfect sectorial structure of the remaining cusps 

 of the trigon. It is more than probable that the loss of this 

 element, which at best is but weakly or imperfectly developed 



