Skeleton of Common Rat. 



2. Skeleton of Common Rat {Mies Decumanus). 



The skeletons of many of the lower Mammalia bear a general 

 resemblance to those of certain quadrupeds lower in the scale of life 

 in such points as the nearness of the level at which their trunk is 

 carried by their limbs to that of the ground on which they move ; 

 and in the maintenance by the long axis of their head, of much the 

 same direction as that of the long axis of their entire trunk. But 

 they invariably present the following distinctive characters, which 

 are as peculiar to the Mammalian class as any of the points fur- 

 nished by the soft parts, such as the blood-cells, the hairy integu- 

 ment, or the mammary glands. In every Mammalian skeleton the 

 lower jaw will be found to be made up of a single mandibular bone 

 on each side, which articulates by a convex facet with the squamosal 

 element of the cranial wall ; and the vertebrae in the trunk always 

 differ from those of the different lower vertebrata in one or more or 

 all of the following points : either in the anchylosis of their several 

 elements, or in the size of their neural canal, or in the shape of 

 the articular ends of their centra, or in the means whereby in the 

 recent state these ai-ticidar ends are brought into relation with each 

 other. In the vertebra of a young mammal the neural arch may 

 not have anchylosed with its centrum ; but in all such cases two 

 discoid epiphyses belonging to the articular ends of the centrum 

 would also remain unanchylosed, as they fuse 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 vertebrae. 

 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 ver- 

 tebra 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 



