VARIOUS FORMS OF LIMBS IN MAMMALS. 151 



PRINCIPAL FORMS OF THE SKELETON IN 

 THE CLASS MAMMALIA. 



In the class Mammalia, which includes the hairj quad- 

 rupeds with the naked apodal whales and biped man, the 

 form of the animal is modified for a great diversity of 

 kinds and spheres of locomotion. Some live exclusively 

 in the ocean, and cleave the liquid element under the form 

 and with the locomotive powers of fishes; some frequent 

 the fresh waters ; some pass a subterraneous existence, and 

 work their way through the solid earth ; some mount aloft, 

 to seek and seize their prey in the air ; some pass their 

 lives in trees ; most, however, dwell on the earth, with 

 various powers of walking, running, and leaping. Lastly, 

 man is modified to sustain his frame erect on the hinder, 

 now become in him the lower, limbs. 



In the Mammalian class, accordingly, we find the limbs 

 progressively endowed with more varied and complicated 

 powers. They retain in the Cetacea (whale and porpoise 

 tribe) their primitive form of flattened fins ; in the Un- 

 gulata (hoofed beasts) one or more of the digits acquire 

 the full complement of joints, but have the extremity en- 

 veloped in a dense hoof; in the Unguiculata (quadrupeds 

 with claws), the limbs, with ampler proportions, have the 

 digits liberated, and armed with claws confined to the 

 upper surface, leaving the under surface of the toes free 

 for the exercise of touch; in the mole, the hand is 

 shortened, thickened, expanded, and converted into a 

 sort of spade ; in the bat, the fingers are lengthened, 

 attenuated, and made outstretchers and supporters of a 

 pair of wings; in the Quadrumana (ape and monkey 



