50 The Skeleton of the Fish 



is extended horizontally, tapering backward, the fin equally 

 divided above and below, without hypural plate. In any form 

 of the tail, it may through degeneration be attenuate or whip-like, 

 a form called leptocercal. 



The Pectoral Limb. The four limbs of the fish are repre- 

 sented by the paired fins. The anterior limb is represented 

 by the pectoral fin and its basal elements with the shoulder- 

 girdle, which in the bony fishes reaches a higher degree of com- 

 plexity than in any other vertebrates. It is in connection with 

 the shoulder-girdle that the greatest confusion in names has 

 occurred. This is due to an attempt to homologize its parts 

 with the shoulder-girdle (scapula, coracoid, and clavicle) of higher 

 vertebrates. But it is not evident that a bony fish possesses 

 a real scapula, coracoid, or even clavicle. The parts of its 

 shoulder-girdle are derived by one line of descent from the un- 

 differentiated elements of the cartilaginous shoulder-girdle of 

 ancestral crossopterygian or dipnoan forms. From a similar 

 ancestry by another line of differentiation has come the am- 

 phibian and reptilian shoulder-girdle and its derivative, the 

 girdle of birds and mammals. 



The Shoulder-girdle. In the higher fishes the uppermost 

 bone of the shoulder-girdle is called the post-temporal (supra- 

 scapula) (53). In the striped bass and in most fishes this 

 bone is jointed to the temporal region of the cranium. Some- 

 times, as in the trigger-fishes, it is grown fast to the skull, but 

 it usually rests lightly with the three points of its upper end. 

 In sharks and skates the shoulder-girdle, which is formed of a 

 continuous cartilage, does not touch the skull. In the eels and 

 their allies, it has, by degradation, lost its connection and the 

 post-temporal rests in the flesh behind the cranium. 



The post-temporal sometimes projects behind through the 

 skin and may bear spines or serrations. In front of the post- 

 temporal and a little to the outside of it is the small supra- 

 temporal (52) also usually connecting the shoulder-girdle with 

 the skull. Below the post-temporal, extending downward and 

 backward, is the flattish supraclavicle (posterotemporal) (54). To 

 this is joined the long clavicle (proscapula) (55), which runs 

 forward and downward in the bony fishes, meeting its fellow on 

 the opposite side in a manner suggesting the wishbone of a 



