EODENTIA. 



813 



also characterize the most specialized Perissodactyla and Artiodactyla among 

 ungulates. The fusion of the inferior part of the fibula with the tibia (found 

 also in the Myomorplia) belongs to the higher types of these orders. The 

 strong intertrochlear ridge of the humerus is an especial feature of the groups 

 mentioned, distinguishing them from the lower types in all the orders. The 

 articulation of the fibula with the calcaneum, mentioned by Mr. Alston, is 

 a character of the Artiodactyla- Associated with these is the elongation of 

 the bones of the limbs, especially the posterior one. The modification of 

 the tarsus in Dipus (the jerboas) evidently has a direct relation to the projec- 

 tile force transmitted through the hind legs in rapid progression by leaping. 

 Here the metatarsals are coossified into a cannon bone, though, as there are 

 three bones involved, the result is somewhat different from the cannon bone 

 of the Buminantia* 



The species of the American Miocenes, including Loup Fork formation, 

 are distributed as follows : 



Hystricope, Leidy , 



Mylagaulus, Cope.. 

 Heliscomys, Cope.. 



Eucastor, Leidy . 

 Castor, L 



Ischyromys, Leidy. 



Meniscomys, Cope 



GymuoptycUus, Cope . 

 Sciurus, Linn 



Htstricomokpha. 



Hyslricidw. 



SCIUKOMOKPHA. 



Mylagaulidoe. 



Fam. ? 

 Castoridce. 



Isehyromyidw. 

 Sciurida. 



White 

 River. 



John 

 Day. 



Loup 

 Fork. 



"On the significance of these characters, see Bulletin U. S. Geolog. Surv. Terrs., 1881, p. 361,, 

 and American Naturalist, 1883, pp. 44-380. 



