GNAWERS OR RODENTS 



(RODENTIA). 



The Rodents have claws on the toes {Unguicnlata) and an incomplete dentition. They have only two large 

 functional incisors, without roots, above and below; there are no canines; the cheek-teeth, all nearly 

 similar in form, stand in a close-set series beside one another, and are separated from the incisors by 

 a wide interval. The placenta is discoidal. 



The Rodents or Gnawers are the order of 

 mammals richest in genera and species. It 

 is an order, the members of which vary in an 

 extraordinary degree through the diversity of 

 secondary characters arising from adaptations 

 to different modes of life, and yet are best 

 marked off from other orders by the con- 

 stancy of their essential characters. The 

 internal structure, on the description of which 

 we cannot here enter, is little different from 

 that of the insect-eaters, and certain details 

 even remind us of the marsupials. 



The rodents are in general small animals; 

 the largest of them, the Capybara, does not 

 exceed in size a one-year-old pig. On the 

 other hand, the smallest rodents rival in 

 diminutiveness the pigmy shrew or shrew- 

 mole of the Irish. With respect to the 

 external characters we may observe a certain 

 parallelism to the insect-eaters, with which 

 the rodents also agree in the simple structure 

 of the brain as well as in the peculiar forma- 

 tion of the sexual organs. 



The dentition presents the chief distin- 

 guishing character, that in which there is but 

 little variation, and in which there is an 

 essential difference from the insect-eaters. 

 In the latter the form, number, and position 

 of the teeth are remarkably varied, while in 



the rodents we must enter into the details 

 of structure to find any distinctions at all 

 between the different types, and even then 

 they are not profound. 



The long-drawn-out jaws have only four 

 incisors altogether, one in each half of each 

 jaw. These incisors have no roots, and 

 consequently keep growing throughout life. 

 They are deep-set in large sockets, which are 

 continued far backwards, and are always 

 curved in the arc of a circle. The enamel 

 layer, often yellow or red in colour, is found 

 only on the outer surface. Sometimes they 

 show longitudinal folds. Since the condyle 

 or articulating surface of the lower jaw is 

 drawn out in the direction of the axis of the 

 skull, and the gnawing action takes place 

 through backward and forward movements 

 of the jaw, these teeth get worn away on 

 the back by mutual friction in such a manner 

 that the enamel layer always presents a 

 chisel-shaped cutting edge transversely placed. 

 The marks which are left, for example, by 

 the teeth of a beaver on trees cut down by 

 them, resemble the marks of a chisel so much 

 that it has often been a matter of controversy 

 whether certain marks found on fragments 

 of timber that have come down from pre- 

 historic times are to be ascribed to the hand 



