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CHAPTER LVIII. 



CLASS MAMMALIA. 



GENERAL STRUCTURE. 



The Mammalia, or highest class of the Vertebrata, are character- 

 ised by having some part of the integument provided with hairs at 

 some period of life, and by the young being nourished for a longer 

 or shorter time by the milk, or special secretion of the mammary 

 glands. As characters available in the case of fossils, it may be 

 observed that the cranium articulates with the atlas vertebra by two 

 occipital condyles, mainly formed by the exoccipitals ; while each 

 ramus of the mandible consists of only a single piece, which proxi- 

 mally articulates directly with the squamosal element of the cranium 

 without the intervention of a quadrate ; and there is no movable 

 joint between the proximal and distal rows of the tarsus. Like the 

 Sauropsida, Mammals possess during development an amnion and 

 allantois, and are totally devoid of gills. They differ from Reptiles 

 and agree with Birds in having a four-chambered heart, warm blood, 

 and a complete double circulation. They are peculiar in that the 

 red corpuscles of the blood are not nucleated and usually circular; 

 in the lungs being freely suspended in the thoracic cavity, which is 

 separated from the abdomen by a muscular partition termed the 

 diaphrag7?i ; in the presence of only the left aortic arch ; and in the 

 perfection of the transverse commissure (corpus callosum) connect- 

 ing the two cerebral hemispheres. Feathers, moreover, are never 

 present, and there is no syrinx or lower vocal organ, although 

 a complete larynx is always developed in the upper portion of the 

 trachea, or respiratory tube. 



It will be unnecessary in this work to make any further mention 

 of the soft parts, but a few remarks must be made concerning the 

 tegumentary and dental systems, and the endoskeleton ; although 

 the student must refer to other works for fuller information on these 

 subjects. With regard to the tegumentary system, it will suffice to 



