814 



THE MIOCENE FAUNA. 



White I John ; Loup 

 River. | Day. ' Fork. 



Myomorpha. 

 MuridcB. 



Eumys, Leidy , 



Hesperomys, Waterh. 

 Paciculus, Cope 



Pleurolicue, Cope . . . 

 Entoptychus, Cope . 



GeomyidoB. 



1 1 



2 ! 



3 I 



5 



Palaeolagus, Leidy. 



Panolax, Cope 



Lepus, Linn 



Lagomorpha. 

 Leporidce. 



The Rodentia, like other divisions of Mammalia, present a succession 

 of changes of structure in time, in the feet and in the teeth. The earliest 

 known forms, as I have pointed out, are the allies of the squin-els, members 

 of the suborder Sciuromorpha. These have the most generalized foot 

 structure because ; first, the trochlear structures of the humerus and tibia 

 are not at all or but little developed ; second, because they have five digits 

 on the feet, and are plantigrade ; and third, because the fibula is not coossi- 

 fied with the tibia. They are similarly primitive in the forms of the teeth, 

 because they are rarely prismatic, and nearly always have long roots and 

 short crowns. The cavy division, or suborder Hystricomorpha, must claim 

 the next place, but many of its members show a decided advance in having 

 a limited number of toes, and prismatic dentition. In the third suborder, 

 Myomorpha (the mice, etc.), we first meet with the coossification of the fibula 

 with the tibia. A good many genera have prismatic teeth, and some of 

 them a restricted number of digits ; and a few of them (the jerboas) even 

 metatarsal bones coosified into a cannon bone. The rabbits have the most 

 specialized characters in all the points mentioned, but they add another 

 character which is most primitive, viz, the presence of four superior incisor 

 teeth. This is probably a remnant of the primitive group from which all 

 the Rodentia have been derived. By the law of homologous groups it is 

 not probable that the divisions of Rodentia were descended from each other, 

 but from corresponding groups of the primary order from which they were 



