.1.) I'l.AlFlSHF.S (HICTIvROSOMATA) 



thfir liixoiiomn \,iliic.' In the systematic part ol this monof^raph the classilicatiou 

 IS boheved to be a more or less natural one, and in the arran(^ement of genera within 

 the subfamilies, and oven of species within the genera, an attempt has been made to 

 show their relationships to one another and to indicate probable lines of descent 

 within the larger ilivisions. Many of the characters used in the definition of genera 

 and species may be shown to be related to special habits, and particularly to the nature 

 of the food and the feeding habits. Many others appear to have no adaptive signifi- 

 cance, but in this connection it may be observed that the habits of very few Flatfishes 

 have been studied, and these are far from thoroughly understood. 



The relative proportions of the various parts of the fish — depth of body, length of 

 head, length of snout, diameter of eye, width of interorbital space, length and depth 

 of caudal peduncle, height of fin-rays, etc., as well as the shape of the head and body, 

 are usually only characters of specific value, but are important in .so far as they 

 provide a means of expressing what may be called the general " form " of the species. 



The position of the nasal organ in relation to the anterior part of the dorsal fin has 

 lieen already considered, and like the structure of the nasal organs themselves, provides 

 a vahiable generic or specific character. The form of the nostrils, and especially of 

 their valves, would appear at first sight to be of taxonomic value, but closer investi- 

 gation reveals considerable variation even within the limits of a single species. In 

 two genera of Paralichthmse, luicitharus and Citharoides, the posterior nostril is much 

 larger than in any other Flatfish, and is covered by an extensive membranous valve 

 extending downwards towards the mouth. These genera, with large deciduous scales, 

 branched dorsal and anal rays, the hinder ra\s twisted .1 little towards the blind side 

 of the caudal peduncle, large mouth with stiimuK iiro]ccting lower jaw and curved 

 mandible, toothed vomer, etc.. occupy an isnLitcd pusition in the subfamily, and bear 

 some resemblance to Lepidorhombiis of the Scophthalmnue, to which they, or Flat- 

 fishes of a similar type, may have given rise. In Eitcitharns and Citharoides the 

 branchial septum is entire, but in the region occupied by the foramen in Lepidnrhombus 

 the partition Ijetween the two branchial cavities is formed only by a fragile membrane. 



The position of the eyes in relation to each other and to the edge of the head is 

 fairly constant in most species, and, due allowance being made for age. and, in some 

 species, for sexual difterences, provides a useful specific character. The presence or 

 absence of rostral or orbital spines, or of rugose prominences or ridges on the inter- 

 orbital or post-orbital regions, may also be of value in distinguishing related species, 

 or may even be used for the definition of genera. The presence of the "upper" eye 

 on the dorsal edge of the head, the development of scales on the surfaces of the 

 eye-balls, and the presence of membranous orbital tentacles, are usually characters 

 of generic importance. 



The size of the mouth, although scarcely deserving the importance gi\'en to it by 

 Giinther (1802, p. 400) and other earlier workers, is, nevertheless, a character of con- 

 siderable importance in the distinction of .species, genera, or even larger groups. Thus, 

 in the subfamily I'leuronectinae the genera mostly fall into two main groups, one in 

 which the mouth is large, or at least of moderate size, and the jaws and dentition 

 nearly equally developed on both sides, and the other in which the mouth is small 

 and asymmetrical, with the jaws and dentition more developed on the blind side. 

 Such genera as Verasper and Clidodernia appear to be transitional between the two 

 groups, and Isopsetta, with a small, asymmetrical mouth and obtuse teeth, bears a 

 marked general resemblance to Psettichthys, a member of the large-mouthed group. 

 In all three subfamilies of Bothida' the jaws and dentition are nearly always more or 

 less ec|iially developed on both sides, although in some of the more specialised genera 

 the teeth are stronger on the blind side. The subfamilies I'aralichthina; and Bothin,f 

 each include one genus — Etropus in the former and Lcrops in the latter — in which the 

 jaws are unequally developed and the teeth almost entirely confined to the blind side. 

 In Chascannpsetta, a deep-water Bothine genus from the Indo-Pacific, the mouth is 



• The S.ileiil.i- .incl CvnoKlosSHl.r ,ir.- not ioii5itlcr«i hprc, as these families will form the subject 

 r,f lh,-se{..n,l VMlutur nf tins work. 



