492 PISCES SUB-CLASS JILTELEOSTOML 



Of comparatively little importance is the small marine family typified by 



the genus Macrurus. Their chief character is that the body terminates 



behind in an elongated, tapering, compressed tail, without 



Family an expanded caudal fin, and is coated with scales, which may 



Macruridce. be either striated, keeled, or spined. Some of the numerous 



species measure as much as a yard in length. 



The last family of the sub-order is that of the flat-fishes, which forms a 

 section to itself, known as the Pleuronectoidei, and characterised by the un- 

 symmetrical conformation of the head and anterior portion 

 Family of the body in the adult. This strange modification renders 



Pleuronectoidce. the flat-fishes quite distinct from all other members of the 

 class, and by bringing the two eyes to one side of the body 

 it may be the right or it may be the left enables them to rest with the other 

 side on the bottom of the sea. Generally, too, they swim in this position, 

 although it is believed that some of the less modified types swira> at least at 

 times, in the normal manner. In this type of fish we have, as Dr. Bashford 

 Dean remarks, " a singular instance of environmental evolution, the flattened 

 body adapting itself both in shape and colour to its bottom-living. Its entire 

 side not the ventral region, as in the rays is flattened to the bottom. The 

 unpaired fins now become of especial value ; they increase in size, and their 

 undulatory movements enable the fish to swim rapidly yet retain its one- 

 sided position ; ventral fins become useless and degenerate. The further 

 adaptations of the flat-fish include its pigmentation only on the upper or 

 light -exposed side, in this giving one of the most remarkable cases of adapta- 

 tion known among vertebrates.' 5 Among the rather numerous genera consti- 

 tuting this family, the one in which the adaptation is least developed is 

 Psettodes, represented by a species whose distribution extends from China to 

 the west coast of Africa. In this fish the dorsal fin, which in the other types 

 commences at, or in front of the eye, begins on the neck ; the eyes being 

 either on the left or the right side. The two species of halibut (Hippoglossus) 

 mark one step in advance, the dorsal fin starting above the eye, although the 

 jaws are still of nearly equal development on both sides. More specialised 

 is the genus Rhombus, which includes the turbot, brill, and mary-sole. Here 

 the dorsal fin commences on the snout, and the eyes are situated on the left 

 side of the body; the scales being either minute or wanting. All these fishes 



have teeth on the vomerine bones 

 of the palate, but since the so- 

 called top-knot of the Mediter- 

 ranean and the Channel has an 

 edentulous vomer, it is referred to 

 a genus apart, under the name of 

 Phrynorhombus. Omitting men- 

 tion of a number of other less 

 familiar genera, brief reference 

 must be made to Pleuronectes, so 

 well known in the shape of the 

 flounder and the plaice. In these 

 Fig. 23. THE FLOUNDER. fish the dorsal fin starts above the 



eye, and the narrow mouth is very 



unsymmetrical, having its teeth much more developed on the blind than on 

 the coloured side. There are no teeth in the palate, and the eyes are gener- 

 ally on the right side, scales being small or wanting. Lastly, the soles (Solea) 



