RODENTIA. 



5S7 



lation of the lower jaw, the condyle of which is placed longi- 

 tudinally and not transversely, so that the jaw slides backwards 

 and forwards. The molars, consequently, have flat crowns, the 

 enamelled surfaces of which are always arranged in transverse 

 ridges, in opposition to the antero-posterior movement of the 

 jaw. The intestine is very long, and the caecum voluminous 



cf - - 



Fig. 228.— A, Skull of the Beaver ([after Owen). B, Diagram of the incisor tooth of a 

 Rodent, showing the chisel-shaped point; a Enamel; ^Dentine. 



'(rarely wanting). The brain is nearly smooth, and without 

 convolutions. The orbits are not separated from the temporal 

 fossffi, and the eyes are directed laterally.. The Rodents are 

 almost all very small animals, and they are mostly very proli- 

 fic. They subsist principally, if not entirely, upon vegetable 

 matters, especially the harder parts of plants, such as the bark 

 and roots. Many of them possess the power of building ela- 

 borate nests, and most of them hybernate. They are very 

 generally distributed over the whole world, but no member of 

 the order has hitherto been detected in rocks older than the 

 Eocene Tertiary. 



The Order Rodentia comprises a very large number of fami- 

 lies, only the more important of which can be noticed here. 



Fam. I. LeporidcE. — In this family are the Hares {Lepus 

 timidus) and Rabbits (Lepus cuniculus), distinguished amongst 

 the Rodents by the possession of two small incisors in the 

 upper jaw, placed behind the central chisel-shaped incisors, so 

 tliat there are four upper incisors in all. The molars and pra;- 

 molars are rootless, and the dental formula is — 



.2—2 0—0 3—3 



m^~=28. 



3—3 



The clavicles are imperfect. The fore-legs are furnished 

 with five toes, and are considerably shorter than the hind-legs, 



