16 RODENTS OF IOWA 



PART I. 



DESCRIPTIONS OF IOWA RODENTS. 



GNAWING MAMMALS. 



Order RODENTIA. 



The order Rodentia is the largest and most widely distributed 

 of the mammalian orders; and, due to the fact that all the members 

 are either herbivorous or omnivorous in diet, it forms a group of 

 considerable economic importance, especially to the agriculturist. 

 In such a large group as this it is but natural to expect great va- 

 riety in form and habits. Some, such as the muskrat, are semi- 

 aquatic; others, such as the pocket gopher and the spermophiles, 

 are more or less fossorial and live in underground burrows; still 

 others, such as the squirrels, are arboreal; while many representa- 

 tives of the group are terrestrial. 



In spite of this great diversity in habits, certain peculiarities are 

 characteristic of the group. A rodent may at once be distinguished 

 from any other mammal by the large, clAsel-like front teeth (in- 

 cisors) , the absence of canines, and the long toothless space between 

 the incisors and the molar series. There is never more than a 

 single pair of lower incisors; and in only one group, the Duplici- 

 dentata, the suborder containing the hares, rabbits and their allies, 

 is the number of upper incisors greater than the lower. In the 

 Duplicidentata a second pair of small incisors is found situated 

 immediately back of the large normal front pair. Due to the fact 

 that these incisor teeth are furnished with enamel on their an- 

 terior faces and also that these teeth continue growing as they be- 

 come worn away, a sharp, chisel-like edge is maintained which is 

 very effective in cutting. Both upper and lower incisors are regu- 

 larly curved, the curvature of the upper pair being somewhat 

 greater than that of the lower pair although the tips of these teeth 

 oppose one another. 



The mouth is divided into two cavities which communicate by a 

 constricted orifice in the space between the incisor and molar teeth, 

 so that the two cavities each contain one of these sets of teeth; 

 the hairy skin of the face is continued inward behind the incisors. 

 Such an arrangement prevents substances not intended for food 



