SCOMBRIFORMES 



463 



The dorsal and anal spines are few and feeble ; there is no subocular 

 shelf. The most distinctive character is seen in the caudal fin : the 

 caudal pedicle is constricted, and the vertebral column ends in a very 

 symmetrical hypural bone ; the fin is deeply forked, and supported by 

 lepidotrichia conspicuously symmetrical about the median axis, with 

 closely approximated bases, forked, and embracing the hypurals, which 



FIG. 490. 



A, skeleton of the tail of Thynnus vulgaris, Cuv. and Val., Tunny fish. B, terminal caudal 

 vertebrae of Scomber scomber, L., Mackerel, c, centrum ; h.a, haemal arch ; hy, hypural com- 

 pound bone, partially concealed in A by the dermal rays ; no, neural arch ; sp, spine. 



they may almost completely conceal (Fig. 490). The tail is often keeled, 

 and the hypural spine strongly developed. The pelvic fins, except when 

 reduced, have the usual single anterior spine, and five lepidotrichia. The 

 scales are cycloid as a rule and tend to disappear. Usually the gill- 

 openings are very wide, the gill - membranes free from the isthmus, 

 the gill-rakers very well developed, the opercular bones unarmed in the 

 adult, the air-bladder large, and the pyloric caeca numerous. 



1. The premaxillae are at most slightly protractile. With a deep 



