74 



FISHES. 



I 



to its extremity, and surrounded by a rayed fringe similar to 

 the vertical fin. A muscle split into 

 numerous fascicles extends all the length 

 of the fin, which is flexible in every part 

 and in every du-ection. The cartilagin- 

 ous framework supporting it is joined to 

 the scapular arch by an oblong cartilage, 

 followed by a broad basal cartilage (a), 

 generally single, sometimes showing 

 traces of a triple division. Along the 

 middle of the fin runs a jointed axis (h), 

 the joints gradually becoming smaller 

 and thinner towards the extremity; 

 each joint bears on each side a three, 

 two, or one-jointed branch (c, d). This 

 axial arrangement of the pectoral skele- 

 ton, which evidently represents one of 

 its first and lowest conditions, has been 

 termed ArcMjjtcrygium by Gegenbaur. 

 It is found in Ceratodus and other 

 genera, but in Zepidosiren the jointed 

 axis only has been preserved, with the 

 addition of rudimentary rays in Pro- 

 topterus. 



The puhic consists of a single 

 flattened subquadrangular cartilage, produced into a long 

 sinole anterior process. Posteriorly it terminates on each 

 side in a condyle, to which the basal cartilage of the ventral 

 paddle is joined. The endoskeleton of the paddle is almost 

 identical with that of the pectoral. 



The Ganoid fishes with persistent notochord, but with a 

 hyostylic skull (that is, a skull with a separate suspensorium) 

 consist of the sub-order Chondrostei, of which the existmg 

 representatives are the Sturgeons {Acijpenser, Sca.phirhynchus, 



Fig. 36.— Fore-limb of 

 Ceratodus. 



