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, Cliary'bdia, 

 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 

 PlatysomaticMliys; 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 deaths (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, Fid. Selsk. Skr. 1863, p. 253, Ann. Mag. Nat. Hist. xv. 1865, 

 p. 361, and Overs. Selsk. Skr. 1876, p. 174 ; Malm, Svensk. Vet. Ak. Eandl. vii. 

 1868, No. i, p. 28 ; A. Agassiz, P. Amer. Ac. xiv. 1878, p. 1 ; Emery, Mitth. 

 Zool. Stat. Neap. iv. 1883, p. 413 ; Facciola, Natural. Sieil. iv. 1885, p. 261, and 

 vi. 1887, p. 74 ; Ehrenbaum, Wiss. Meeresunters. (2), ii. 1897, p. 255 ; Nishikawa, 

 Annot. Zool. Japan, i. 1897, p. 73. 



