366 THE VERTEBRATE SKELETON. 



and are much more thickly coated with enamel on their 

 anterior face than elsewhere ; consequently, as they wear down 

 they acquire and retain a chisel-shaped (scalpriform) edge. 

 There is never more than one pair of incisors in the mandible, 

 and except in the Hares and Rabbits, there is similarly only a 

 single pair in the upper jaw. These animals are, too, the only 

 rodents which have well developed deciduous incisors. There 

 is always a long diastema separating the incisors from the 

 grinding teeth. The grinding teeth, which are arranged in a 

 continuous series, vary in number from two to six in the upper 

 jaw, and from two to five in the lower jaw. The number of 

 premolars is always below the normal, often they are altogether 

 wanting, but generally they are ^. Sometimes the grinding 

 teeth form roots, sometimes they grow persistently. 



The premaxillae are always large, and the orbits always com- 

 municate freely with the temporal fossae. The condyle of the 

 mandible is elongated from before backwards, and owing to 

 the absence of a postglenoid process to the squamosal, a back- 

 ward and forward motion of the jaw can take place. The 

 zygomatic arch is complete, but the jugal is short and only 

 forms the middle of it. The palate is small, being some- 

 times, as in the hares, narrowed from before backwards, 

 sometimes as in the mole-rats (Bathyerginae) narrowed trans- 

 versely. 



The thoraco-lumbar vertebrae are usually nineteen in 

 number. Clavicles are generally present, and the acromion 

 of the scapula is commonly very long. The feet are as a rule 

 plantigrade, and provided with five clawed digits. 



There are two main groups of Rodentia ; the Duplici- 

 dentata, or Hares and Rabbits, which have two pairs of upper 

 incisors, whose enamel extends round to the posterior surface ; 

 and the Simplicidentata, in which there is only a single pair 

 of upper incisors, whose enamel is confined to the anterior 

 surface. This group includes all the Rodents except the 

 Hares and Rabbits. 



