CLASS 5. MAMMALS (Mammalia) 



Warm-blooded, air-breathing vertebrates which have a hairy integu- 

 ment. With the exception of the Australian monotremes all mammals 

 are viviparous, and all possess mammary glands on the ventral body 

 surface with which they suckle their young, in the marsupials these 

 glands being located in an integumental pouch. The teeth are adapted 

 to the grasping and chewing of food, there being four different forms, 

 the incisors or front teeth, the canines, and the premolars and molars 

 which are the back teeth or grinders. The heart is composed of four 

 chambers, two ventricles and two auricles; there is a single (left) aortic 

 arch and the blood contains non-nucleated red blood-corpuscles. The 

 head articulates with the neck by two occipital condyles and there are, 

 with rare exceptions (the Florida manatee alone of American mammals) , 

 seven cervical vertebrae. The lower jaw consists of a single bone on 

 each side and articulates directly with the cranium and not by means of 

 the quadrate bones. An external ear in form of a large integumental 

 fold is, with the exception of certain aquatic mammals, present. The 

 cochlea is spirally coiled, and the tympanum is joined with the inner 

 ear by means of the three ear-ossicles. The abdominal cavity is sepa- 

 rated from the thoracic by means of a muscular diaphragm. 



On the Identification of Mammals. — The measurements of a 

 species of mammals are given in millimeters and include (i) the length 

 of the body from the tip of the snout to the hinder end of the vertebrae 

 of the tail, (2) the length of the tail measured from its base to the hinder 

 end of its vertebrae and (3) the length of the hind foot measured from 

 the bend of the ankle to the tip of the longest toe. An average large 

 male specimen is probably generally used in making the measurements, 

 but it must be remembered that individual differences always exist 

 between individuals of a species, due to differences in age, sex, locality 

 and environmental conditions, and that a considerable margin must be 

 allowed for these differences. The color of a species is also to a certain 

 extent variable, and is much more so in some species of mammals than 

 in others. 



The general form of the body and that of its various parts are usu- 

 ally noted, and also any peculiarities of form or color which would tend 

 to mark a species or larger group. The dentition, or number of teeth, 



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