CLASSIFICATION 



43 



among existing Flatfishes, and it is of some interest to note that Atheresihes resembles 

 Pseltodes, not only in this character, but also in having the "upper" eye on the dorsal 

 surface of the head and most of the teeth in the jaws barbed. In all other characters, 

 however, Atheresthes is a typical Pleuronectid, and an examination of the optic nerves 

 shows it to be a truly dextral form. It is very doubtful, therefore, whether the resem- 

 blances to Psettodes indicate close relationship with that genus. In the Pcecilopsettince, 

 formerly included in the Pleuronectmae, the lamina; are comparatively few in number, 

 well raised from the basal membrane, and radiate from a central rachis. In the 

 Paralichthodina: the laminae are likewise arranged transversely to or radiate from a 

 rachis. In the Samarina; the nasal organs are very small, the laminae being only 

 slightly raised, but these have the parallel arrangement (without rachis) charac- 

 teristic of the Pleuronectina. Finally, in the subfamily Rhombosoleinae both types 

 of arrangement of the laminae are found, and the central rachis may be shown to have 

 been lost within the subfamily. ^ In the more generalised forms (e.g. Oiicopterus, 

 Psammodiscus, Pelotretis, Azygopus) the laminae are arranged in pinnate form with a 

 long central rachis, in Colisiiiim they radiate from a very short rachis, and in Ammo- 

 tretis, Peltorhamphns and Rhombosolea the rachis has been lost and the laminae have the 

 parallel arrangement of the Pleuronectinae. 



To summarise the above, the order Heterosomata may be divided into five families. 

 Of these, the sinistral Bothidae and the dextral Pleuronectidae may each be further 

 subdivided into three and five subfamilies resepctively. The relationships of the 

 various families and subfamilies are indicated in the following diagram : 



SCOPHTHAOIIN.E 



PARALICHTHIN^ 



PLEURONECTIX.E 



PCECILOPSETTIX.E 



SAMARIN^ 



PARALICHTHODIX.E 



RHOMBOSOLEIN.E 



CYNOGLOSSID.C 



HETEROSO.MATA 



PERCOIDEA 



B. GENERIC AND SPECIFIC CHARACTERS. 



The characters used for the distinction of genera and species are necessarily either 

 external or of such a nature that they can be observed without elaborate dissection. 

 These are too numerous and varied to be dealt with in any detail, but it may be of 

 interest to consider briefly some of the more important structural modifications and 



' Norman (i9::6b, p. .'58, fig.). 



