273 



CHAPTER XXXVIII. 



MAMMALIA. 

 GrENERAL CHARACTERS OF THE MAMMALIA. 



The last and highest class of the Vertehrata, that of the 

 Mammalia, may be shortly defined as including Vertebrate 

 animals in lohicli some part or other of the integument is al- 

 ways provided with haii's at some time of life ; and the young 

 are nourished for a longer or shorter time, hy means of a spe- 

 cial fluid — the milh — secreted hy special glands — the mammary 

 glands. These two characters are of themselves sufficient 

 broadly to separate the Mammals from all other classes of 

 the Vertebrate sub-kingdom. In addition, however, to these 

 two leading peculiarities, the Mammals exhibit the following 

 other characters of scarcely less importance : — 



1. The skull articulates with the vertebral column by 

 means of a double articulation, the occipital bone carrying 

 two condyles, in place of the single condyle of the Eejotiles 

 and Birds. 



2. The lower jaw or mandible consists of two halves or 

 rami, united anteriorly by a symphysis, but not necessarily 

 anchylosed ; but these are each composed of a single piece, 

 instead of being complex and consisting of several pieces, 

 as in the Eep tiles and Birds. Further, the lower jaw al- 

 ways articulates directly with the squamosal element of the 

 skull, and is never united to an os quadratum, as in the 

 Sauropsida. 



3. The two hemispheres of the cerebral mass, or brain 

 VOL. II. S 



