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DIVISION III. MAMMALIA. 



CHAPTER LXXII. 

 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 longei' 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-king- 

 dom. 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 

 anchylosed ; 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 

 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 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 

 chambers, 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. The red blood-corpuscles are non- 

 nucleated, and, with the exception of the Camelidce, they are 

 circular biconcave discs. There is only one aorta the left 

 which turns over the left bronchus, and not over the right, as 

 it does in Birds. 



