XXII ACANTHOPTERYGII 685 
sometimes absent; if fully developed and normally formed, the 
bones of the pectoral and pelvic girdles as in the Zeidae. Ventral 
fins usually with 5 to 7 soft rays. 
Scales usually imbricate, cycloid or ctenoid; rarely absent ; 
bony tubercles sometimes present. Air-bladder absent. 
Most species, and even genera, are either sinistral or dextral, 
but this is inconstant in some, including the most generalised 
genus, Psettodes. The very young are transparent and sym- 
metrical, with an eye on each side, and swim in a vertical position 
like other Fishes. These larval forms have been described as 
distinct genera, under the names of Peloria, Bibronia, Charybdia, 
etc. As they grow, the eye of one side moves by degrees to the 
other side, where it becomes the upper eye. If at that age the 
dorsal fin does not extend to the frontal region, the migrating 
eye simply moves over the line of the profile, temporarily assuming 
the position which it preserves in Psettodes, Atheresthes, and 
Platysomatichthys; in other genera, the dorsal fin has already 
extended to the snout before the migration takes place, and the 
eye, passing between the frontal bone and the tissues supporting 
the fin, appears to pass from side to side through the head, as 
was believed by some of the earlier observers.! 
Flat-fishes are a large group of some 500 species, mostly 
marine, a few species related to the Soles being confined to the 
fresh waters of South America and the Malay Archipelago. They 
range from the Arctic Circle to the southern coasts of the 
Southern Hemisphere; many occur at great depths (Citharichthys 
dinoceros down to 955 fathoms). Well-preserved remains 
referred to Psetta occur in the Upper Eocene, and a species of 
Solea is known from the Lower Miocene. 
A satisfactory classification of the Pleuronectidae is still a 
desideratum, and cannot be attempted until the osteology of the 
very numerous forms has been thoroughly studied. Even the 
division into two principal groups, regarded by some recent 
authors as families, Pleuronectidae and Soleidae, is based on 
1 Cf. Steenstrup, Vid. Selsk. Skr. 1863, p. 253, Ann. Mag. Nat. Hist. xv. 1865, 
p. 361, and Overs. Selsk. Skr. 1876, p. 174; Malm, Svensk. Vet. Ak. Handl. vii. 
1868, No. 4, p. 28; A. Agassiz, P. Amer. Ac. xiv. 1878, p. 1; Emery, Mitth. 
Zool. Stat. Neap. iv. 1883, p. 413; Facciola, Natural. Sicil. iv. 1885, p. 261, and 
vi. 1887, p. 74; Ehrenbaum, Wiss, Mceresunters. (2), ii. 1897, p. 255 ; Nishikawa, 
Annot. Zool. Japan. i. 1897, p. 73. 
