360 CLASSIFICATION OF VERTEBRATES. 



hand, is more normal, except in the cetacea, where it is apparently 

 absent, the one or two rib-like bones which occur imbedded in 

 the muscles free from the vertebral column being usually inter- 

 preted as femur and tibia. In all others the ilia are united to 

 the sacral vertebrae, while with rare exceptions the pubes of the 

 two sides, and usually the ischia as well, unite in a symphysis 

 below. In monotremes and marsupials, marsupial bones, prob- 

 ably epipubic in character (p. 171), are developed from the 

 cephalic side of the pelvic girdle. 



In the skeleton of the free appendages great variations occur, 

 especially in the direction of reduction of digits, etc.; and for 

 details of these reference must be made to the accounts of the 

 separate orders. In general it may be said that the tendency 

 is towards a reduction in the number of digits, and towards an 

 alternation and interlocking of the carpal and tarsal bones. In 

 the swimming-forms there is also a shortening of the limbs, 

 the reduction going so far that in the case of the sirenia only the 

 elbow joint is functional, while in the whales even this joint is 

 lost. On the other hand, in these forms, as in the ichthyosaurs 

 and plesiosaurs, an increase in the number of phalanges is more 

 or less marked. In the bats, on the other hand, the elongation 

 of most of the digits of the hand, and their utilization as sup- 

 ports of the wing, is noticeable. 



In detail : the humerus may be either long or short, the con- 

 ditions here being usually in reversed correlation to those found 

 in the metacarpus. The ulna and radius are usually longer 

 than the humerus, the ulna being produced beyond the hinge of 

 the elbow as an olecranon process. The radius is more closely 

 related to the carpus, and is capable of turning more or less 

 freely around the ulna in the process of pronation and supination 

 of the manus. Occasionally radius and ulna coalesce. 



The femur usually bears two or three enlargements (tro- 

 chanters) for the attachment of muscles. At the knee joint a 

 patella or knee-pan usually occurs. The tibia and fibula are 

 usually longer than the femur, and in the marsupials are capable 

 of marked rotation. On the other hand, the fibula is frequently 

 reduced, and united more or less closely to the tibia. 



The greatest variations occur in the carpus and tarsus and 



