CHARACTERS OF THE RODENTS 141 



PART II. 

 DIAGNOSTIC CHARACTERS OF THE RODENTS. 



In this more technical part of the bulletin is included a diag- 

 nosis of the order Rodentia, following which are keys and brief 

 diagnostic descriptions of the suborders, families, and genera in 

 regular sequence. Tables are included also lor use in delimiting 

 the species and subspecies of rodents known to occur, or to have 

 occurred in the state, these tables being intended only for the Iowa 

 representatives of the group. The page number following the 

 specific name in the keys, refers to the general discussion of the 

 species in part I. 



Order RODENTIA. Gnawing Animals. 



Incisor teeth long, strong, growing from persistent pulps, the 

 enamel chiefly or wholly on the anterior face, producing a chisel- 

 like -cutting edge ; canine teeth absent ; molar teeth few, and sep- 

 arated from the incisors by a wide diastema; toes usually with 

 nail-like claws ; feet usually plantigrade ; cascum very large ; brain 

 smooth or with few furrows, the cerebrum not overlapping the 

 cerebellum; articular surface for lower jaw elongated antero-poster- 

 iorly ; malar bone in middle of zygomatic arch ; size small to moder- 

 ate; furred, some forms spined. 



Key to Suborders of Rodeijtia. 



Never more than two upper incisors ; enamel confined to anterior 

 faces of these teeth Simplicidentata 



Upper incisors four in number, the second pair very small and 

 situated immediately behind the first; enamel extending partly on 

 sides of these teeth Duplicidentata 



Suborder Simplicidentata. 



One pair upper incisors, the enamel confined to anterior faces; 

 incisive foramina of palate moderate and distinct; bony palate 

 extensive ; tibia and fibula distinct, the latter not articulating with 

 the calcaneum. 



