484 ZOOLOGY SECT. 



five distal carpals are represented by the trapezium, trapezoid, 

 magnum, and unciform, the last being the equivalent of the 

 fourth and fifth distalia. There are never more than five digits, 

 and in many forms the number is greatly reduced ; only in certain 

 Cetacea does the number of phalanges in a digit ever exceed 

 three. 



The three elements of the pelvic arch unite to form a single 

 bone, the innominatum. The ilia unite by broad surfaces with 

 the sacrum the pubes unite in a pubic symphysis. All three 

 usually take a share in the formation of the acetabulurn, but 

 the pubis may be shut out by a small cotyloid bone. In the 

 shank the inner or tibial element is always the larger ; the fibula 

 may be rudimentary. A large sesamoid bone the patella is 

 almost universally formed in close relation to the knee joint. In 

 the tarsus there are two proximal bones, the astragalus and cal- 

 caneum, 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 row of 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 sup- 

 ported 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 

 oh 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 a grasping organ. 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 the 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 



