156 



COMPARATIVE ANATOMY 



has taken place. 1 This is also true as regards the pectoral fin, in 

 which an additional basal piece, or mesopterygium, is usually present 

 (Fig. 99) : there may even be four basalia. These complications 

 arise in connection with the greater importance of the pectoral 

 fin as an organ of locomotion. The distal portions of both fins 

 are supported by horny fibres (cf. note on p. 137). With the 



a, 



Raff 



FIG. 117. RIGHT PELVIC FIN OF 

 Heptanckus. From the ventral 

 side. 



BP, pelvic plate ; Fo l , f, nerve-for- 

 amina ; Pr, propterygium ; 1'ml, 

 radii, which show secondary seg- 

 mentation ; S /'(!,, basi- or meta- 

 pterygium. 



FIG. 118. PECTORAL Fix OF Cera- 

 todus fosteri. 



a, 1>, the two first segments of the 

 main axial ray ; FS, dermal rays, 

 shown only on one side ; f, f, 

 lateral rays. 



exception of one or at most of very few all the rays are 

 situated on the same side of the basalia (uniscrial type). 



In Rays, the propterygium of the pectoral fin, and usually also the 

 metapterygium, are strongly developed, the former extending far 

 forwards so as to be connected with the skull by ligament, and in 

 some cases even uniting with its fellow in front of the skull. 



Dipnoans. The cartilaginous pectoral and pelvic fins are here 



1 In male Elasmobranchii a number of pieces of cartilage are connected witli 

 the distal snd of the basipterygium of the pelvic iin as a support for the 

 copulatory organs or claspers (q. n. ) : these may become more or less calcified. 



