ACANTHOPTERYGII 683 



forming part of the family out of which the latter have sprung, 

 were it not that they have lost the last half-gill. Ampliistium 

 is probably more nearly related to the Pleuronectidae, which may 

 have been directly derived from the family of which it is as yet 

 the only known representative.^ 



This division embraces three families only :— 



A distinct spinous dorsal fin ; anal spines detached from the soft portion ; 

 a ventral, spine ; gills three and a half, four slits between them 



1. Zeidae. 

 Dorsal and anal spines few, continuous with the soft raj's ; a ventral spine 



2. Amphistiidae.\ 

 No spines ; cranium twisted in front, with the two orbits on one side ; 



gills 4, a slit behind the fourth ... 3. Pleuronectidae. 



Fam. 1. Zeidae. — No subocular shelf ; praemaxillaries strongly 

 protractile. G-ill-membranes free from isthmus ; 7 or 8 branchio- 

 stegal rays ; gills 3-^ ; pseudobranchiae well developed. Lower 

 pharyngeal bones separated. Vertebrae 30 to 46, the anterior 

 with sessile ribs, the posterior praecaudals with long neural spines 

 bent forwards and with transverse processes directed downwards, 

 forming haemal arches and bearing the ribs at their extremity; 

 epipleurals much reduced or absent ; hypural large, without the 

 basal spine or knob present in most Perciformes and all Scombri- 

 formes and Percesoces, bearing fewer than 20 rays. Dorsal and 

 anal fins elongate, the former with a distinct spinous portion, the 

 latter with 1 to 4 spines detached from the soft portion. 

 Pectoral fin supported by 4 pterygials, of which 3 are in 

 contact with the perforated scapular bone ; post-temporal forked 

 and solidly attached to the skull. Ventral fin with 1 spine and 

 6 to 8 soft rays. 



Scales small or minute, sometimes hard and rough and firmly 

 joined in vertical series ; bony plates may be present along the 

 base of the vertical fins. Air-bladder present. 



Twelve species are known from the Atlantic and Pacific Oceans, 

 referable to 5 genera: Grammicolepis, Cyttus, Gyttoi^&is, Zenion, 

 and Zeus. Oreosoma was founded on a young form of a fish 

 allied to Cyttus. Eemains of Zeiis occur in the Oligocene, and 

 Cyttoides, from the same period, has been compared with Cyttus. 



The well-known John Dory (Zeus faher) is much valued for 

 the table. 



1 Cf. Boulenger, Ann. Mag. Nat. Hist. (7), 1902, p. 295, and C. R. Ac. Sci. 

 cxxxvii. 1903, p. 523. t Extinct. 



