400 RODENTS. 



steel laid upon the back of a chisel ; whence the name, * dentes 

 scalprarii,' given to the incisors of the Rodentia.(l) 



The varieties to which these incisors are subject in the different 

 Rodents are limited to their proportional size, and to the colour 

 and sculpturing of the anterior surface. Thus in the Guinea-pig, 

 Jerboa, and Squirrel, the breadth of the incisors is not half so 

 great as that of the molars, whilst in the Coypu they are as broad, 

 and in the Cape Mole-rats, (Bathyergus and Orycteromys) , broader 

 than the molars. 



In the Coypu, Beaver, Agouti, and some other Rodents, the 

 enamelled surface of the incisors is of a bright orange or reddish 

 brown colour. In some genera of Rodents as OrycteromySy Otomys, 

 Meriones, Gerbilla, Hydrochwrus, Lepus and Lagomys, the anterior 

 surface of the upper incisors is indented by a deep longitudinal 

 groove. This character seems not to influence the food or habits 

 of the species ; it is often present in one genus and absent in 

 another of the same natural family : in most Rodents the anterior 

 enamelled surface of the scalpriform teeth is smooth and uniform. 



The molar teeth are always few in number, obliquely implanted 

 and obliquely abraded, the lateral series converging anteriorly in 

 both jaws ; but they present a striking contrast to the incisors 

 in the range of their varieties, which are so numerous that they typify 

 almost all the modifications of form and structure which are met 

 with in the molar teeth of the omnivorous and herbivorous genera 

 of other orders of Mammalia. 



In some Rodents the molar teeth are rootless, like those of the 

 Wombat, the Toxodon and Elasmothere ; some have short roots 

 tardily developed, like the molars of the Horse and Elephant ; and 

 some soon acquire roots of the ordinary proportional length. (2) 



(1) John Hunter grouped together the quadrupeds composing this order under the name 

 of ' Scalpris-dentata ;' but the large curved, chisel-shaped incisors, though common to all the 

 Rodents, are not peculiar to that order : we have seen them in the Wombat amongst the 

 Marsupials ; they are present in the Cheiromys amongst the Lemurine group of Quadrumana, 

 and in the Toxodon amongst the Pachyderms ; some of the Shrews have the anterior incisors 

 restricted in number and developed in bulk, almost to their proportions in the Rodentia. 



(2) Prof. Erdl has given a tabular arrangement of the molars of the Rodentia, according 



