xm PHYLUM CHORDATA 501 



patella is almost universally formed in close relation to the knee- 

 joint. In the tarsus there are two proximal bones, the astragalus 

 and calcaneum, the latter undoubtedly corresponding to the fibulare 

 of the carpus of lower Vertebrates, and the proximal part of the 

 former to the intermedium and its distal portion to the proximal 

 of the two central elements present in the tarsus of some Amphibia. 

 The scaphoid or navicular represents the second central bone, and 

 the distal tarsals are 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 accord- 

 ance with 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 quadrupedal 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 of the 

 quadrupeds have the extremities prehensile, the hand and foot 

 being converted into grasping organs. This is most marked in 

 quadrupeds 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 extending 

 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. 1139) the 



