SKELETON OF COMMON RAT. 5 



with it at a later period than the neural arch, and they furnish a mark as 

 distinctive of the Mammalian class as any other connected with the verte- 

 brae. Some mammals have an opisthocoelian ball and socket articulation 

 between the centre of their vertebrae ; and the crocodiles resemble the 

 mammals in having interarticular fibrocartilaginous discs to connect their 

 ball and socket centre-joints instead of synovial joints ; but in such cases 

 the greater size of the neural canal or the absence of neurocentral sutures, 

 or the absence of sutures between the body and the lateral processes, 

 would enable us, without having recourse to a microscopic examination of 

 the bony tissue, to identify a vertebra as having belonged to a mammal. 

 In all mammals, except the Cetacea, the maximum number of phalanges in 

 any one digit is limited to three ; in nearly all, the number of cervical 

 vertebrae is 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 possession of 

 the pairs of scalpriform incisors in the upper and lower jaws, from which 

 their name is taken. There is a single pair of incisors in the upper jaw in 

 all Rodents, except the Leporidae, which are hence called ' Duplicidentatil 

 as having 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 living 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 im- 

 portance in the conformation both of the skull and of particular bones. 

 The intermaxillaries, in relation with which ' the upper incisors are first 

 developed, and which form a great part of their permanent sockets, 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 living Rodents, as in the 

 Elephants, they interpose between the nasals and the maxillaries, whilst 

 failing themselves to reach the lacrymals. The maxillary bone gives origin 

 on the concave surface of its malar process to a large part of the masseter 

 muscle, but a more deeply placed part of the muscle passes behind or inside 

 of that process and takes origin from the sides of the snout. The presence 

 of this deep head to the masseter is peculiar to but by no means constant 

 in Rodents, varying with the infra- or ant-orbitally placed canal through 

 which it passes. It co-operates, by passing round the back of the malar 



