FAMILY CASTORID^ 145 



Genus Cynomijs Rafinesque. 



Body unmarked, not sharply bicolor ; pelage short ; tail short, 

 flat ; ears rudimentary ; cheek pouches shallow ; wrist and heel 

 heavily furred ; feet with claws on all five toes, the thumb large, 

 and the nail well developed ; po>storbital processes strong:, de- 

 curved : infraorbital foramen larg'e, subtriangular ; molar series 

 divergent anteriorly, the teeth large, and expanded laterally; 

 first premolar large. 



Dental fcrmula—l. VI- Pm. 2/1; M. 3/3=22. 



A single form occurs in Iowa. 



Black-tailed Prairie-dog. 

 Cynomj/s ludovicianus ludovicianus. p. 40 



Genus Marmota Blumenbach. 



Size large, legs short ; tail short, thickly haired ; front feet with 

 four well developed toes, the thumb rudimentary and with a flat 

 nail; hind feet with five toes; :?kull heavy, flattened above; post- 

 orbital processes broad at base, stout, deeurved, at right angles to 

 long axis of skull ; first premolar nearly as large as second ; molar 

 series with two transverse grooves across their crowns ; nearly 

 parallel. j j ' 



Dental formula^l. VI ; Pm. 2/1 ; M. 3/3=22. 



A single form occurs in Iowa. 



Southern Woodchuck. 

 Marmota nionax monax. p. 45 



Family CASTORID^. 

 BEAVERS. 



Tail broad and flattened, hairless and covered Avith scales ; five 

 toes on both fore and hind feet, the latter broadly ^rebbed ; second 

 toe of hind foot with double claw; skull stout, without postorlital 

 processes; infraorbital foramen nearly concealed by a large, almost 

 vertical ridge on the maxilla; incisors large, powerful, the lower 

 much longer than the upper, and the anterior surface deep orange- 

 red in color; molars single-rooted with re-entering enamel folds, 

 and decreasing in size posteriorly ; molar series converging ante- 

 riorly ; palate arched and contracted anteriorly. 

 10 



