146 RODENTS OF IOWA 



Genus Castor Linnaeus. 



In addition to the family characters above mentioned, the fol- 

 lowing may be included here, since but a single genus is recog- 

 nized: Upper molars subequal, with one inner a,nd tw'o outer 

 enamel folds; molars with dentinal pulp persisting until late in 

 life; teeth of molar series not parallel but converging anteriorly. 



Dental formula I. 1/1 ; Pm. 1/1.; M. 3/320. 



A single form has occurred in Iowa. 



Beaver. 

 Castor canadensis canadensis. p. 50 



Family MURID/E. 

 BATS AND MICE. 



Lower incisors compressed; premolars wanting; molars never 

 more than three in each jaw, with or without roots, tuberculate 

 or with enamel folds; frontals contracted; infraorbital foramen 

 wider above than below; jugal splintlike; postorbital processes 

 wanting; tibia and fibula anchylosed below. 



Key to the Genera of Muridae. 



1. Total length 10.00 inches or over 6 



Total length less than 10.00 inches 2 



2. Front of upper incisors with longitudinal groove 3 



Front of upper incisors without longitudinal groove 4 



3. Superior incisors with a deep longitudinal groove nearly 



as broad as the face of the tooth Reithrodontomys 



Superior incisors longitudinally grooved on outer edges 



only Syna-ptomys 



4. Tail short ; length of tail vertebrae usually not over 2.00 



inches ; form short ; ears short Microtus 



Tail longer ; length of tail vertebrae over 2.00 inches ; form 

 longer; ears longer. ..'..- 5 



5. Molars of upper jaw with tubercles on crowns extending 



in two longitudinal rows Per omy sous 



Molars of upper jaw with tubercles on crowns extending 



in three longitudinal rows Mus 



6. Tail rounded Eattus 



Tail laterally compressed Fiber 



