ii INTRODUCTION 



enjoy the most numerous faculties, the most delicate sensationsj the 

 most varied powers of motion. Their organization is more differ- 

 entiated than in any other Vertebrates, and they have a more per- 

 fect combination of powers. They are inferior to Birds in muscular 

 movement, their respiration being less in amount, and the circula- 

 tion less rapid, on which account their demand for food is not 

 so constant. 



The generality of them are terrestrial, some inhabit trees, others 

 burrow in the ground, whilst a few can fly, and some are perfectly 

 aquatic. With such difFerences in habit we of course find corres- 

 ponding differences in external structure. The anterior extremities 

 in Bats are lengthened to support the flying membrane, whilst 

 in Whales they are shortened and fin-like, and the terminal points 

 of the phalanges vary from sharp raptorial claws, to the solid hoof 

 of the Horse, or the flat pad of the Camel and Elephant. On these 

 characters, combined with those of teeth, are founded the difi^rent 

 orders of Mammals. 



The form of the body varies, but we can generally distinguish 

 the head, neck and trunk, and most have caudal appendages. The 

 head varies greatly in its form and proportions, as does the ear, 

 and from these also characters of more or less importance are drawn ; 

 but the teeth, in form, number, difierences, and relative position, 

 afford the most varied, prominent, and decisive characters, as well for 

 the Orders of Mammals as for genera, and even sometimes species, 

 and require more lengthened notice than any other point in the ex- 

 ternal anatomy of Mammals. 



Teeth being used by Mammals both to seize and collect food, and 

 to reduce it to a fit state for swallowing, their form furnishes a 

 clue to the instincts and habits of the animal. They are placed in 

 a single series along the edge of the upper and lower jaw bones, 

 so as to oppose each other, and are always fixed in cavities called 

 sockets or alveoli, an arrangement which elsewhere is only found in 

 the Crocodiles among Reptiles* 



Ivory or Dentine, forms the sole material of some simple teeth, 

 as in some Cetacea, and in the tusk of the Elephant j but in most 

 •teeth another substance of crystalline texture and extremely hard 



