XXIII 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. Amphistiwm 
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 rays ; a ventral spine 
2. Amphistiidae.t 
No spines ; cranium twisted in front, with the two orbits on one side; 
gills 4, a slit behind the fourth . : ; . 38. Pleuronectidae. 
Fam. 1. Zeidae.— No subocular shelf ; praemaxillaries strongly 
protractile. Gill-membranes free from isthmus; 7 or 8 branchio- 
stegal rays; gills 34; 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 5 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. 
Seales 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, Cyttopsis, Zenion, 
and Zeus. Oreosoma was founded on a young form of a fish 
allied to Cyttus. Remains of Zeus occur in the Oligocene, and 
Cyttoides, from the same period, has been compared with Cyttus. 
The well-known John Dory (Zeus faber) is much valued for 
the table. 
1 Cf. Boulenger, Ann. Mag. Nat. Hist. (7), 1902, p. 295, and C. R. Ac. Sei. 
CXXXvii. 1903, p. 523. + Extinct. 
