CLASSIFICATION 37 



It is quite clear from what has been said in discussing the origin and evolution 

 of the Flatfishes that Psettodes, the most generalised of existing Flatfishes, is sharply 

 marked of) from all other members of the order and must rank as the type of a distinct 

 family. The principal characters distinguishing the Psettodida; are as follows : the 

 occurrence of dextral and sinistral individuals in equal numbers, the dimorphic optic 

 chiasma, the posterior origin of the dorsal fin, the presence of spinuous rays in the 

 dorsal and pelvic fins, the large, symmetrical mouth, with well-developed supple- 

 mental maxillaTv. the strong teeth, the toothed palatines, the normal urohyal with the 

 lower edge scarcely curved, the nearly symmetrical position of the nasal organs, and 

 the small number of vertebrae (24). Psettodes shares most of these characters with its 

 perch-like ancestors, and, as Regan (1920, p. 324) has pointed out, it may have retained 

 so many Percoid features "because it has not adopted progression along the bottom 

 by undulating movements of the body and marginal fins to the same extent as other 

 flat-fishes." 



In all the remaining families the dorsal fin extends forward on to the head at least 

 to above the eye, all the fin-rays are articulated, and the number of vertebrae 

 is never less than 28. This increase in the number of vertebra, which reaches 

 its maximum in the Cynoglossidae, some of which may have as many as 70, 

 is difficult to explain in terms of any of the orthodox theories of evolution. That 

 increase in the number of muscle segments (the number of vertebrae being, of course, 

 determined by the number of myomeres) must be a mechanical advantage to a fish 

 which has taken to swimming by undulating movements of the whole body cannot 

 be denied. At the same time, in view of the fact that the number of vertebrse in 

 Psettodes, as well as in a number of families of Percoid fishes, seems to be rigidly fi.xed 

 at 24, it is difficult to see how natural selection can be evoked to explain the increase 

 in number in more specialised Flatfishes. "We are almost compelled to believe." 

 writes Regan (1926, p. 85), " that muscular movements, the efforts of a fish to swim 

 m a certain way, may lead to an alteration in the number of muscle segments of its 

 descendants . 



The " Soles ", as represented by the families Soleidae and Cynoglossidae, have 

 generally been regarded as forming a distinct group, but most authors have looked 

 upon them as degraded " Flounders ". It is true that these fishes are, in many 

 respects, highly specialised, but nearly all the more superficial characters in which 

 they differ from the " Flounders " can be shown to be adaptive, and to be correlated 

 with their special habits, such as the method of locomotion, burying themselves in 

 sand or mud by day and seeking their prey mainly by night by the use of the senses 

 of smell and touch, and so on. There appear to be very few genera of Bothidae or 

 Pleuronectidae that could fairly be described as leading in the direction of the Soleidae 

 or Cynoglossidae, and, in spite of the dearth of positive characters to define the two 

 groups, the line of demarcation between the " Soles " on the one hand and the 

 " Flounders " on the other is a definite one. Without a detailed study of the two 

 families concerned, it is difficult to come to any definite conclusions about the origin 

 of the " Soles ". I would suggest, however, that these famihes have not been derived 

 either from the Bothidae or Pleuronectidae, but that both the dextral Soleidae and the 

 sinistral Cynoglossidae have probably sprung independently from generahsed Psettodes- 

 like stock. The presence of a dimorphic optic chiasma in these fishes led Parker 

 (1903, p. 235) to the same conclusion, and he regarded the " Soles " as a natural group, 

 and the " degenerate descendants of the original stock of flatfishes that had not yet 

 passed beyond the stage of dimorphic chiasmata." The fact that the nasal organs 

 of the " Soles " are symmetrically placed as in Psettodes, whereas, in most other 

 Flatfishes that of the bhnd side has accompanied or followed the eye in its migration 

 and lies nearly on the median line of the head, provides further confirmation of this 

 view. Finally, the structure of the eggs and the form of the post-larva;' in the 

 Soleidae and Cynoglossidae again suggests that they have been derived from primitive 

 Flatfishes rather than from the " Flounders ". The small mouth, with the lower jaw 



• See Kyle (1921). 



