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BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Tot. VI 



terranean, others are arboreal, and. some live on the surface of the 

 ground. Of the latter, some are provided with formidable spines as a 

 protection against enemies, while others depend for their safety on their 

 speed. Of the latter character are the Leporidce of the Lagumorpha, and 

 1 wish to note how that they have superadded to the ordinary rodent 

 structure certain points which also characterize the most specialized 

 Perissodaetyla and Artiodactyla among ungulates. The fusion of the infe- 

 rior 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, distinguish- 

 ing 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 

 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 

 I)rojectile 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 dif- 

 ferent from the cannon bone of the Euminantia. 



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

 are distributed as follows: 



White 

 Eiver. 



Truckee. 



Loup 



Fork. 



Hystrix, L . 



Mylaganlus, Cope . 

 Heliscomys, Cope . 



Eucastor, Leidy. 

 Castor, L 



Ischyromys, Leidy . 



Meniscomys, Cope — 

 Gymnoptychus, Cope 

 Sciurus, Linn 



Eumys, Leidy 



Hesperomys, Waterh 

 Paciculus, Cope 



Plenrollcus, Cope . . 

 Entoptychns, Cope. 



Palseolagus, Leidy . 



Panolax, Cope 



Lepus, Linn 



Htstbicomorpha. 

 Hygiricidce. 



SCIMOMORPHA. 



MylagaulidcB. 



Fam. t 



Castoridce. 



IschyromyidcB. 

 Seiuridce. 



Mtomoepha. 

 Muridce. 



Geomyidce. 



Lagomoepha. 

 Leporidce. 



