6 Descriptions of Preparations. 



neither more nor less than seven ; and the number of the lumbar 

 vertebrae is never less than two. There are very rarely any verte- 

 brae with unanchylosed ribs anteriorly to the first dorsal vertebrae. 

 The jaws are ordinarily dentigerous^ but teeth are never found 

 elsewhere than upon the mandibular^ maxillary, and intermaxillary 

 bones; the grinding teeth very frequently have more than a 

 single root or fang, a method of implantation never observed in 

 any other class. 



The most distinctive character of the Rodent order is the posses- 

 sion of the pairs of scalpriform incisors in the upper and lower jaws, 

 from the functions of which their class-name is taken. There is a 

 single pair of incisors in the upper jaw in all Rodents, except those 

 of the family Leporidae, in which there are two pairs placed one 

 behind the other, the hinder pair being the smaller. In the lower 

 jaw there is a single pair only in all Rodents, without exception. 

 The upper incisors form a larger segment of a smaller circle, the 

 lower a smaller segment of a larger circle. The peculiarities of 

 their growth, which goes on uninterruptedly during the life of the 

 creature from a persistent pulp, and of their functions, entail 

 changes of great importance in the general conformation of the 

 skull and of particular bones. The intermaxillaries, in relation 

 with which the upper incisors are first developed, and which form 

 a large part of the sockets in which they are permanently lodged, 

 are larger in relation to the rest of the skull and of the animal than 

 in perhaps any other mammals ; — they form the whole, or nearly the 

 whole, of the sides and under surface of the bony snout, and in all 

 Rodents they shut off the nasals from contact with the maxil- 

 laries. The maxillary bone, besides forming- part of the socket for 

 the lodgment of the teeth, furnishes in its malar process a point of 

 origin for a deeply-placed part of the masseter, which co-operates 

 very strongly with the temporal muscle in moving the lower jaw 

 in a vertical direction, and bringing its incisors into play upon those 

 of the upper jaw ; whence probably the inverse ratio which has 

 been observed to obtain between the temporal and the antorbital 

 fossae is to be accounted for. The masseter muscle arises from 

 nearly the whole length of the malar arch, which is made up ordi- 

 narily of the malar process of the maxillary, of the malar bone, and 

 of the malar process of the squamosal, and sometimes of the lacry- 

 mal also. It is by the contraction of those of its fibres which pass 



