38 FLATFISHES (HETEKOSOMATA) 



never prominent, the jaws of the bhnd side strongly cuned and toothed, the absence 

 of a free jirrcopercular margin, the absence of ribs, etc., are all characters which are 

 shared by the Soleida? and Cynoglossidae, but in spite of these resemblances, it seems 

 doubtful whether the two families are really very closely related.' 



Mention may be made of certain Australian and New Zealand genera of the 

 I'leuroncctid subfamily Rhombosoleinrc (Ammntretis, Colislium, Peltorhamphus), some 

 of the species of which exhibit a striking general resemblance to members of the Soleida>. 

 This resemblance, which is, perhaps, most marked in Colislium, is shown by the general 

 form of the body, the shape of the head, particularly its priEorbital part, the small 

 eyes, the symmetrical nasal organs, the strongly curved jaws of the blind side, the 

 absence of teeth in those of the ocular side, and the extension of the dorsal fin to the 

 end of the snout. The development of membranous folds on the blind side of the 

 rays of the marginal fins, the modification of many of the scales on the blind side of 

 the head to form filamentous processes, and the fringed lower lip, are other Soleid 

 features.- Many of these characters, however, are probably correlated with nocturnal 

 habits, or with the habit of burrowing in sand or mud, and have almost certainly 

 been evolved within the subfamily. It seems improbable that any of the above- 

 mentioned genera are closely related to the " Soles ", which they superficially resemble, 

 but provide a striking example of parallelism.^ 



The work of Parker was later than that of Jordan and his collaborators and that 

 of Kyle, and these workers did not appreciate the importance of right- or left-sidedness 

 m the " Flounders ", sometimes including dextral and sinistral genera within the same 

 subfamily. The monomorphism of the optic chiasma in these Flatfishes, unaffected 

 by the occurrence of reversed individuals in certain species, is of primary importance, 

 and the differences in the structure of the eggs m dextral and sinistral forms (see p. 30) 

 provides further evidence in favour of the division of the " Flounders " into two 

 families, one dextral, the other sinistral. The arrangement of the olfactor\' laminae 

 in the nasal organs, originally believed to be another character separating the two 

 families,^ has been shown to be of only generic importance.'' 



The sinistral family Bothidae may be subdivided into three subfamilies, distin- 

 guished mainly by the form of the pelvic fins (Fig. 24) and the structure of the vertebrae. 

 In the Parahchthma; (a) the pelvic fins are usually both short-based, although in some 

 genera that of the ocular side is somewhat extended ; its anterior ray, however, is 

 never far m advance of the first ray of the pelvic fin of the blind side. The two fins 

 are symmetrical or that of the ocular side is nearly median in position. The pelvic 

 fins are supported by the pelvic bones and are placed behind the cleithra.* In the 

 vertebral column all, or nearly all, the praecaudal vertebra; have the parapophyses 

 downw'ardly directed, and these are either united directly with one another or con- 

 nected by bridges to form closed haemal arches. There are no transverse apophyses in 

 the caudal vertebrae. In the second subfamily, Bothinae, the pelvic fin of the blind 

 side has a short base as in the Parahchthina>, but that of the ocular .side has become 

 elongate, and is placed along the median line of the body ; the first ray of this fin is 

 situated well in advance of that of the fin of the blind side (b). The elongate pelvic 

 fin IS supported by a cartilaginous plate which is placed in advance of the cleithra, 

 and extends forward to the urohyal ; the fin of the blind side is supported by the 

 pelvic bone as in the Parahchthinie. The praecaudal vertebrae have the parapophyses 

 connected or united as in the previous subfamily, but well-developed transverse 



* Another char.ictcr which distinguishes both the Soleido' and Cynogiossid.r from all the 

 " Flounders " is the absence of a post-cleithrum in the pectoral arch. 



' See Norman (1926B, p. Zjq). 



^ A study of the osteology of the Rhombosoleina-. and of the optic chiasma in tlie \ arious genera, 

 would be of considerable interest. I'nfortunately, the material in the Bntisli .Museum is insufficient 

 for such an investigation. 



* See Regan (ioiob, p. 489). 



^ See Norman {i(j2bu, p. 25*^). 



« For a full account of the pelvii; hns and associated structures see Kyle (i.joob. pp. 340-343)- 



