Order RODENTIA. 



The Orders hitherto noticed follow each other in fairly natural 

 sequence. The Rodents, comprising squirrels, marmots, rats and 

 mice, jerboas, porcupines, hares and their allies, have no connexion 

 with the Chiroptera, and but little with any other order of mammals. 

 Like the Insectivora and Chiroptera, the Eodentia are mostly 

 animals of small size. 



Eodents can be readily distinguished by their dentition. There 

 are no canines, and there are always two large, chisel-shaped 

 rootless incisors, with the anterior surface curved, in front of each 

 jaw. There are never more than two incisors in the lower jaw ; in 

 the upper there are, in one suborder, additional small incisors 

 behind the anterior pair, not at the side of them as usual. The 

 large incisors, or " rodent tusks " as they are sometimes termed, 

 grow throughout the lifetime of the animals, and have long basal 

 portions extending into deep alveoli. The incisors are widely 

 separated from the grinding-teeth. Premolars may be present or 

 absent. There are almost always three true molars in each side 

 of each jaw ; the crowns of these molars are in most cases tuber- 

 cular, and, when worn, traversed by laminae of enamel. 



The most important character of the skull is the large si/e of 

 the premaxillary bones, which contain the elongate basal portions of 

 the upper incisors, and completely separate the nasals from the 

 maxillaries. The orbits are never circumscribed by bone, and post- 

 orbital processes are generally wanting. A zygomatic arch is 

 always present. The condyle of the mandible is longitudinal, 

 so that the lower jaw works backwards and forwards. 



The mouth is divided into an anterior cavity containing the 

 incisors, and a posterior cavity containing the molars, the two 

 connected by a constricted orifice, lined partially or wholly by the 

 hairy integument of the face. The feet are plantigrade, or semi- 

 plantigrade, usually pentadactyle, and unguiculate. The intestine 

 has a large caecum (except in Myoxidce) ; the uterus is double or 

 two-horned, and the placenta discoidal and deciduate. In many 

 families the females have an elongated perforate clitoris in front 

 of the genital orifice, and may easily be mistaken for males. The. 

 testes in most rodents (not in the Duplicidentata) are retained in 

 the abdomen, except in the rutting-season, when they become 

 greatly enlarged. The cerebral hemispheres are smooth and do 

 not extend back over any part of the cerebellum. 



Rodents are cosmopolitan in distribution, and comprise more 

 species than any other Mammalian order. Nearly all are exclusively 



