SKELETON I 23 



The same groups of nonplacental mammals are characterized by the 

 presence of marsupial bones (fig. 132). These are preformed in 

 cartilage and extend forward from either pubis in the ventral abdom- 

 inal wall. Their homology is very uncertain; but they are not the 

 ypsiloid of the urodeles (p. 119). 



The Free Appendages 



These are of two kinds, the paired fins (ichthyopterygia) of the fishes 

 and the legs or their modifications (chiropterygia) found in all classes 

 of tetrapoda. The former is merely a mechanism for altering the 



Tig. 133. — Diagrams illustrating theories of origin of appendages. A, B^C, origin 

 of biserial appendage (C) from gill arch {A)\ D, biserial appendage (archipterygium) ; 

 E, F, evolution of elasmobranch fin; G, dotted lines indicate parts involved in origin of 

 leg from fin; H, dotted parts show another view of origin of elements of leg. 



position of the body in the water, and requires a small amount of 

 flexibility, being moved as a whole. The assumption of terrestrial 

 habits necessitates the support of the body above the ground and its 

 propulsion. Hence the chiropterygium must have a firmer skeleton, 

 with at the same time joints for motion and intrinsic muscles to move 

 the parts on each other. The chiropterygium was undoubtedly 

 derived from the fish fin, but the problem of how the change was 

 made has not been solved. Only paleontology can give the answer. 



There are two views as to the origin of the chiropterygium, both based upon 

 the loss of certain parts and the persistence of others in a modified form. One 

 view assumes the persistence of a basal as the framework (humerus or radius) of 

 the upper limb. Two proximal radials as that of the next limb segment, while 

 the skeleton of ankle and foot is derived from a corresponding number of distal 



