120 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 ( Pfemur 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 CCELOM (BODY CAVITIES). 



The ccelom includes all of the primitive cavities, right and left, 

 enveloped by the mesothelium (p. 10). With the division of the 



