XV. 



progression employed by various quadrupeds, according to the 

 amount of palmar and plantar surfaces in contact with the 

 ground ; thus those, as the Bears, which use the whole of 

 the lower surface of the carpus, metacarpus, and phalanges, 

 are termed "plantigrade"] those, as the Horse, which apply the 

 distal surface of the ungual phalanx and the horny structures 

 surrounding it, " unguligrade" ; intermediate forms exist, such 

 as "phalangigrade" for the Camel, and " subplantigrade " for 

 most Carnivores. In the Bats the digits are enormously elongated 

 for the purpose of supporting a cutaneous expansion constituting 

 the organs of flight ; while in the Whales and Dolphins the manus 

 is formed into a paddle, covered by continuous integument, and 

 without any trace of nails or claws, and the number of phalanges 

 belonging to the second and third digits always exceeds the 

 normal number in Mammals, and sometimes very considerably. 



Posterior limb. The posterior limb consists of a pelvic girdle, 

 and three segments belonging to the limb proper, the thigh 

 (femur) , the leg (tibia and fibula), and the foot (pes) . 



Pelvic girdle. The pelvic girdle is present in some form in all 

 Mammals, but in the Sirenians and Cetaceans it is in a very 

 rudimentary condition. Each half of this girdle consists of three 

 bones, which in the adult state are ankylosed together into a 

 solid mass, and the single bone thus formed is technically known 

 as the " os innorninatum." Of the three sections of which this 

 bone is composed, the upper (ilium) is firmly articulated to the 

 sacrum, and of the two lower, the antero-inferior f pubis) forms a 

 symphysis with its fellow of the opposite side, except in the 

 Insectivorous genera, Soricidrr, Talpidfc, and Ckrysochloridcr, 

 where these bones are entirely separated, while the postero- 

 inferior f ischium) are never united. These three sections unite 

 around a cup-shaped cavity, the acetabulum, into which the 

 proximal end of the femur is received. Between the pubis and 

 ischium there is a large opening known as the "obturator foramen." 

 The two innominate bones, in conjunction with the sacral, form 

 the pelvis. By this direct articulation of the innominate bones 

 with the vertebral column, greater strength is given to the 

 hind limbs to increase their powers of supporting and propeling 

 the body. In the Monotremes and Marsupials an additional 

 pair of thin, flat, elongated bones, called the " epipubic " or 

 " marsupial " bones, the exact function of which is but im- 

 perfectly understood, is attached to the fore part of the pubis, 

 and projects forward into the muscular wall of the abdomen. 



Thujli and Ley. As with the fore-limb, the proximal segment 

 of the hind limb has but one bone, the femur, and the second 

 two, the tibia and the fibula : these lie parallel to one another, 

 the former, which is much the more strongly developed being on 

 the inner side and more to the front, while the latter is on the 

 outer side and behind. They are never either permanently 



