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CHAPTER LXII. 



CLASS MA MM A L I A— continued. 



Orders Rodentia and Carxivora. 



Order VII. Rodentia. — The Rodents form one of the best-defined 

 orders of the whole class, and are readily characterised by the ab- 

 sence of canines, and their chisel-like (scalpriform) incisors (of 

 which there is only a single lower pair', separated by a long 

 diastema from the cheek-teeth (fig. 1298), and of which generally 

 only the anterior face is coated with enamel. The dentition is 

 diphyodont and heterodont ; the incisors grow from persistent 

 pulps, and the cheek-teeth may be either rooted or rootless. The 

 crowns of the latter are entirely adapted for grinding, and are very 

 frequently (fig. 1292) complicated by deep infoldings of enamel. 



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 The premolars are very generally either - or — , and the true molars 



1 o 



<■> 



-. The premaxillae are large ; the zygomatic arch is complete, the 

 3 



middle portion being generally formed by the jugal ; the orbit is 

 confluent with the temporal fossa ; there is a long diastema ; and 

 the condyle of the mandible is antero-posteriorly elongated, and, 

 through the absence of a postglenoid process in the cranium, 

 capable of a backwards and forwards motion. The feet are 

 generally partially or entirely plantigrade, and usually furnished 

 with five unguiculate digits. More or less complete clavicles are 

 present, and the acromion of the scapula frequently has a long 

 recurving process. All the existing forms are of comparatively 

 small size, and most of them are of terrestrial habits. The feature 

 of hypsodontism, which we have already noticed under the head of 

 the Ungulata, attains its greatest development in this order ; its final 

 stage being the production of rootless cheek-teeth. 



That the Rodents are connected in some manner with the more 



