Evolution of the Order Heterosomata. 491 



Order HETEROSOMATA. 



Asymmetrical, with both eyes on one side. Body strongly 

 compressed, with the praecaudal region short ; dorsal and anal 

 fins long; caudal fin with 17 principal rays (15 branched) or 

 fewer ; pelvic fins 6-rayed or less, thoracic or jugular, with 

 the pelvic bones directly attached to the cleithra. Air-bladder 

 absent in the adult. Mouth more or less protractile, bordered 

 above by the prsemaxillaries only. Parietals separated by 

 the supra-occipital ; interorbital bar mainly formed by the 

 frontal of eyed side ; frontal of blind side extending to prse- 

 frontal external to upper eye ; no orbito-sphenoid. Pectoral 

 arch attached to skull by a forked post-temporal ; no meso- 

 coracoid. Vertebral column of solid centra coossified with 

 the arches; posterior pracaudal vertebra? with downwardly 

 directed parapophyses. 



Suborder 1. PsETTODOIDEA. 



Dorsal fin not extending forward on the head ; anterior 

 dorsal rays spinous ; each pelvic fin of a spine and 5 soft 

 rays. Maxillary with a well-developed supra-maxillary bone ; 

 palatines toothed ; uroliyal normal, the lower edge scarcely 

 curved. Two post-cleithra on each side. Vertebra? 24 

 (10 + 14). Species with sinistral and dextral individuals 

 equally numerous and with the optic chiasma dimorphic. 



Family 1. PsettodidsB. 



Pelvic fins nearly symmetrical in form and position, poste- 

 rior to the cleithra. Mouth large, with strong pointed teeth ; 

 jaws and dentition equally developed on both sides. Nasal 

 organ of blind side scarcely higher than the other; olfactory 

 lamina? arranged transversely to or radiating from a central 

 rachis. Prsecaudal parapophyses downwardly directed and 

 united to form closed haemal arches ; pectoral radials well 

 developed. 



A single species, Psettodes erumei, ranging from West 

 Africa to China. It has no gill-rakers, and the strongly 

 toothed mouth is larger than in any other flat-iish ; this is 

 evidently a predaceous fish, which probably lies on the 

 bottom concealed from its prey, and then darts out, swimming 

 rapidly for a short distance by lateral movements of the tail. 

 Probably it has retained so many Percoid features because it 

 has not adopted progression by undulating movements of the 

 body and marginal fins to the same extent as other fishes of 

 this order. 



33* 



