THE EODENTIA. 435 



With the exception of one group of Rodents, there are only 

 two teeth in the premaxillse ; and these have the same cha- 

 racters as the incisors of the mandible. The Lagomorpha, or 

 Hares and Rabbits, however, have a second pair of incisors 

 of small size, behind the first, in the upper jaw. The molars 

 are from two to six in number, in each haK of the ixpper 

 jaw, and two to iive, in the lower jaw. They consist of 

 enamel, dentine, and cement, and their crowns may be 

 tuberculate or laminate in pattern. Sometimes they form 

 roots, but, in other cases, they grow throughout life. 

 Where there are more than three grinding teeth, the 

 one which precedes the three hindermost has displaced a 

 milk tooth ; but where the grinding teeth are fewer than 

 three, or only three, none of them displaces a milk tooth. 

 Even when milk teeth exist they may be shed before birth, 

 as in the Guinea-pig. 



The premaxillary bones are always large, and the orbits 

 are never shut ofi' by bone from the temporal fossa. Very 

 generally, the condyle of the mandible is elongated from 

 before backwards. 



With the exception of one group, the Dormice {Myox- 

 inoe), all Rodents have a large coecum. 



The cerebral hemispheres leave the cerebellum largely 

 uncovered, when the brain is viewed from above. They 

 are either smooth externally, or very moderately convoluted. 

 The corpus callosum is well developed. 



With the exceptions noted, the foregoing characters are 

 universal among the Bodentia. There are other peculiari- 

 ties which are generally i^resent, and when they exist, are 

 very characteristic, though they are not universal. 



Thus the dorso-lumbar vertebrse are usually nineteen in 

 number. There is a large interj)arietal ossification. The 

 jugal bone is comparatively short, and occupies only the 

 middle of the zygomatic arch. 



The clavicles are very generally present ; though whoUy 

 absent in some genera, as, for example, the Guinea-pig, 

 {Cavia). The acromion commonly sends a process back- 

 wards over the infra-spinous fossa. There is a ninth bone 



