LI^IBS 



159 



FIG. 121. PECTORAL Fix OF 

 Polypterus. 



The form of the pelvic fin in Polypterus and other bony 

 Ganoids may be easily derived from that seen in the cartilaginous 

 representatives of this order, and it 

 1 may be assumed that the basale is -{- 



due to the concrescence of a larger 

 number of separate radii, which are, 

 therefore, much less numerous than 

 in the Sturgeons (Fig. 104). Bony 

 rays support the distal part of both 

 pairs of fins (p. 137). 



Tdeosts. A still further reduc- 

 tion has taken place in the primitive 

 skeleton of the paired fins in Tele- 

 osts, there being at most only a few 

 radials articulating with the arch 

 (Fig. 100), and even these (especi- 

 ally in the case of the pelvic fin, in 

 which the arch is usually considered 

 to be undifferentiated), may be want- 

 ing. The main part of each fin is 

 supported by bony rays, as in Ganoids. 

 The skeleton of the fins of Siluroids, 

 Cyprinoids, and Gymnotidse conies 

 nearest to that of Ganoids. 



FS, bony dermal rajs ; Nl, nerve 

 foramina ; Oss, centre of ossi- 

 fication in MS ; Pr, Mt, bony 

 marginal rays, which meet at t, 

 so that the intermediate region 

 ( MS) does not reach the arch ; 

 Ra, Ra l , radii. 

 Though it is possible to derive 



the skeleton of the fin of all the Orders of Fishes from a single 

 ground -type, it is a far more difficult task to trace the connection 

 of the latter with the extremities of Amphibia and Amniota. 

 Between these two types of extremity there seems to be a wide 

 gap, in consequence of the different conditions of life existing 

 between aquatic and terrestrial Vertebrates. We do not know 

 how the pentadactyle limb of an air-breathing Vertebrate (cheiro- 

 pterygium), adapted for progression upon land, has been derived 

 from the fin (ichthyopterygium), only fitted for use in the water, 

 and Palaeontology has so far furnished no solution to this problem. 

 There is, however, a certain amount of probability in the view 

 that the cheiropterygium has arisen from such an ichthyopterygium 

 as that seen in cartilaginous Fishes, although it is quite un- 

 certain as to how far the individual parts are comparable to one 

 another (Fig. 122), and how the fin, which is practically a single- 

 jointed lever, amply sufficient for the movement of the body in a 

 fluid medium, became gradually transformed into a many-jointed 

 system of levers. 



PAIRED LIMBS OF THE HIGHER 

 VERTEBRATA. 



