MAMMALIA. 397 



CHAPTER XXXV. 



MAMMALIA. 

 GENERAL CHARACTERS OF THE MAMMALIA. 



THE last and highest class of the Vertebrata, that of the Mam- 

 malia, may be shortly defined as including Vertebrate animals 

 in which some part or other of the integument is always provided 

 with hairs at some time of life; and the young are nourished for 

 a longer or shorter time, by means of a special fluid the milk 

 secreted by 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 con- 

 dyles, in place of the single condyle of the Reptiles and Birds. 



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

 rami, united anteriorly by a symphysis, but not necessarily an- 

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

 instead of being complex and consisting of several pieces, as 

 in the Reptiles and Birds. Further, the lower jaw always ar- 

 ticulates 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 proper, 

 are united together by a more or less extensively developed 

 " corpus callosum " or commissure. 



4. The heart consists as in Birds of four cavities or cham- 

 bers, two auricles and two ventricles. The right and left sides 

 of the heart are completely separated from one another, and 

 there is no communication between the pulmonary and systemic 

 circulations. 



5. The cavities of the thorax and abdomen are completely 

 separated from one another by a muscular partition the dia- 

 phragm or midriff. 



6. The respiratory organs are in the form of two lungs placed 

 in the thorax, but none of the bronchi end in air-receptacles, 

 distributed through the body, as in Birds. 



As regards the Osteology of the Mammals, the following 

 points should be noticed : 



With the exception of the Whales and Dolphins (Cetacea), 



