122 



COMPARATIVE ANATOMY 



-a, 



FREE LIMBS. 

 Fishes and Dipnoans. 



In the following description the pelvic fin will be considered 

 before the pectoral, as it usually retains a simpler and more 

 primitive form. 



,$ Elasmobrancliii and Holocephali. The cartilaginous skeleton of 

 the fins is the most richly segmented in these Fishes. There 



are usually two main elements (basalia) 

 in the pelvic fin which articulate with 

 the arch and with which a variable 

 number of segmented rays (radii) are 

 connected, the latter passing towards 

 the periphery of the fin (Fig. 88). Both 

 the larger, posterior main element (basi- 

 or metapterygiiim), and the smaller, 

 inconstant propterygium must, as al- 

 ready stated (p. 105), be looked upon 

 as originating phylogenetically, at any 

 rate by a fusion of the proximal ends 

 of the primary cartilaginous rays of the 

 fin; and the form and relations of 

 these main elements vary according to 

 the degree in which such a fusion 

 has taken place. 1 This is also true 

 as regards the pectoral fin, in which an 

 additional basal piece, or mesoptery- 

 gium, is usually present between the 

 pro- and metapterygia, and, like these, 

 articulates with a special convexity on 

 the pectoral arch (Fig. 83) : there may 

 even be four basalia. These compli- 

 cations arise in connection with the 

 greater importance of the pectoral than 

 the pelvic fin as an organ of locomotion. 

 The distal portions of both fins are 

 supported by horny fibres (p. 103). 

 With the exception of one (Fig. 83, t) 

 or at most of very few all the rays are 

 situated on the same side of the basalia 

 (uniserial type). 



Dipnoi. The cartilaginous pectoral and pelvic fins are here 

 also essentially similar to one another, the latter being 

 rather the simpler of the two. From a segmented main-ray or 



1 In male Elasmobranchii and Holocephali a number of pieces of cartilage are 

 connected with the distal end of the metapterygium of the pelvic fin as a support 

 for the copulatory organs or claspers : these may become more or less calcified. 



FIG. 101. PECTORAL FIN OF 



Ceratodua fosteri. 



a, b, the two first segments of 

 the main axial ray ; t, t, 

 lateral rays ; FS, horny 

 rays, shown only on one 

 side. 



