I20 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
nearly typical, but there is a marked power of rotation of the foot and 
especially of the hand by the motion of the lower end of the radius 
around the ulna. There also the appendages may form grasping 
organs, both features being found to a less extent in several lower groups. 
In the bats digits II to V are greatly elongated (either metacarpals or 
phalanges may be lengthened) to support the wing, the first digit remain- 
ing normal. In the whales and sirenians the basal parts of the fore 
limb are greatly shortened, while there is a multiplication of the pha- 
langes, recalling that of the plesiosaurs. The hind limb is entirely 
lacking in the sirenians and some of the whales; in other whales there 
are two vestigial bones (?femur and tibia) imbedded in the muscles of 
the trunk. 
The mammalian humerus is frequently perforated by a (supra- or 
entepicondylar) foramen passing through the inner lower end, a 
feature found elsewhere only in some theriomorphs. In many un- 
gulates the ulna is reduced and may be fused with the radius; elsewhere 
it is well developed. Even where reduced it always bears on its prox- 
imal end a strong olecranon process, extending beyond the elbow- 
joint for the attachment of the extensor muscles of the lower limb. 
The femur bears a varying number (up to three) of prominences or 
trochanters for the attachment of muscles. The fibula resembles the 
ulna in its tendency to reduction. The patella (p. 118) at the knee- 
joint is analogous to the olecranon process, though it never joins the 
other bones. 
The details of the modification of the feet cannot be described here. 
The ankle-joint is never intratarsal but always between tarsal and 
crural bones. ‘There is considerable variety in the extent to which the 
bones of the feet rest upon the ground. In the plantigrade foot, as in 
the bear and man, the sole of the foot includes the metapodial bones? in 
the digitigrade forms, like the dog and cat, the sole includes only the 
distal phalanges, while in unguligrades (cow, horse) the weight of the 
body is supported on the hoofs (p. 27) developed on the upper (ante- 
rior) surface of the distal phalanges. There is frequently a reduction 
of the digits, reaching its extreme in the horse where only digit III 
persists in a functional condition. 
THE C@LOM (BODY CAVITIES). 
‘The ccoelom includes all of the primitive cavities, right and left, 
enveloped by the mesothelium (p. 10). With the division of the 
