516 ZOOLOGY SECT, xin 



central bone, and the distal row of tarsals is represented by the 

 cuboid and the three cuneiforms. 



The external form of the limbs and the mode of articulation of 

 the bones vary in the various orders of the Mammalia, in accordance 

 with differences in the mode of locomotion. In most the habitual 

 attitude is that which is termed the quadrupedal the body being 

 supported in a horizontal position by all four limbs. In quadru- 

 pedal Mammals the manus and pes sometimes rest on the ventral 

 surfaces of the entire metacarpal and metatarsal regions as well 

 as on the phalanges when the limbs are said to be plantigrade ; 

 or on the ventral surfaces of the phalanges only (digitigrade) ; or 

 on the hoofs developed on the terminal phalanges (unguligrade). 

 Many quadrupeds have the extremities prehensile, the hand 

 and foot being converted into grasping organs. This is most 

 marked in those that pass the greater part of their life among 

 the branches of trees, and in the Sloths the modification goes 

 so far that both hands and feet are converted into mere hooks 

 by means of which the animal is enabled to suspend itself, body 

 downwards, from the branches of trees. 



Certain Mammals, again, have their limbs modified for locomo- 

 tion through the air. The only truly flying Mammals are the Bats 

 and the so-called " Flying Foxes," in which the digits of the fore- 

 limb are greatly extended so as to support a wide, delicate fold of 

 skin constituting the wing. In other so-called flying Mammals, 

 such as the Flying Squirrels and Flying Phalangers, there is no 

 active flight, and the limbs undergo no special modification ; the 

 flying organ, if it may be so termed, in these cases being merely a 

 parachute or patagium in the form of lateral flaps of skin ex- 

 tending along the sides of the body between the fore- and hind- 

 limbs. 



Further, there are certain groups of swimming Mammals. Most 

 Mammals, without any special modification of the limbs, are able 

 to swim, and some of the quadrupeds, such as the Tapirs and 

 Hippopotami, spend a great part of their life in the water. But 

 there are certain Mammals in which the limbs are so specially 

 modified to fit them for an aquatic existence assuming the form 

 of flippers or swimming paddles that locomotion on land becomes 

 almost, if not quite, impossible. Such are the Whales and 

 Porpoises, the Dugongs and Manatees, and, in a less degree, the 

 Seals and Walruses. 



Skeleton of Prototheria. In the Prototheria (Fig. 1127) the 

 epiphyses of the vertebrae are not well developed in the Platypus, 

 being represented only in the caudal region, and they appear to 

 be absent in Echidna. In both genera there is the normal number 

 of vertebrae in the cervical region. The odontoid process long 

 remains separate from the centrum of the axis. The cervical 

 transverse processes are separately ossified, and only completely 



