FAMILY CASTORID^] 145 



Genus Cynomys Rafmesque. 



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; postorbital processes strong 1 , de- 

 curved: infraorbital foramen large, subtriangular ; molar series 

 divergent anteriorly, the teeth large, and expanded laterally; 

 first premolar large. 



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



A single form occurs in Iowa. 



Black-tailed Prairie-dog. 

 Cynomys ludovicianus ludovicianus. p. 40 



Genus Marmcta Blumenbach. 



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

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

 nail; hind feet with five tees; skull heavy, flattened above; post- 

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

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

 series with two transverse grooves across their crowns; nearly 

 parallel. j 



Dental formula^-I. 1/1 ; Pm. 2/1 ; M. 3/3=22. 



A single form occurs in Iowa. 



Southern Woodchuck. 

 Marmota monax monax. p. 45 



Family CASTORID/E. 

 BEAVERS. 



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

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

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

 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 



