678 TELEOSTEIL CHAP. 
inserted low down. A single genus, Rhachicentrum (Hlacate), 
with a single species from the coasts of the tropical and 
warmer parts of the Atlantic and of the Indian Ocean. 
Fam. 3. Scombridae.—Praemaxillaries large, not protractile, 
beak-like. Vertebrae 30 to 50, without transverse processes, 
but some of the hinder praecaudals with haemal arches; ribs 
inserted on the centra or on the haemal arches when these are 
present ; epipleurals all on the centra. Scales eycloid and usually 
very small (except in (astrochisma), sometimes absent. <A 
spinous dorsal fin formed of slender spines, folding into a sheath ; 
soft dorsal longer and broken up into finlets, similar to the anal; 
pectoral inserted high up the sides. Hypural bones completely 
embraced by the forked bases of the caudal rays. Pseudo- 
branchiae present. 
Fic. 415.—Tunny (Thunnus thynnus). (After Cuvier and Valenciennes.) x 4. 
About 50 species, referred to the following genera :—Scomber, 
Avuis, Thunnus, Sarda, Cybium, Acanthocybium, Gastrochisma 
(Lepidothynnus). Numerous fossil representatives in Tertiary 
beds, belonging to Scomber, Auaxis, Thunnus, Cybium, and to the 
extinct genera Hothynnus, Isurichthys, Palimphyes, Scombrinus, 
Sphyraenodus, Scombramphodon. 
These fishes, elegant in form and often in colour, are among 
the swiftest of the inhabitants of the sea. Some are migratory, 
like the Mackerel (Scomber scombrus) of the North Atlantic, 
whilst others are remarkable for their wide distribution. The 
Tunny (Thunnus thynnus), for instance, the largest member of 
the family, reaching a length of 10 feet, inhabits the Atlantic, 
Pacific, and Indian Oceans, extending as far north as the British 
