of the Fishery Board for SrotianeL 



the wide -mouthed species, and separated from the Plaice and its 

 allies ms small-mouthed species. Such a classification very obviously 

 distorts the real affinities, because in all other important characters 

 the Plaice and Halibut groups come together and separate them- 

 selves widely from the Turbots. Nevertheless, if we disregarded 

 all consideration of affinities, and endeavoured empirically to find 

 those characteristics which displayed best the general resemblances 

 and differences, this would be one of the characters chosen. It is 

 of the most widespread general nature, and there are very few transi- 

 tional forms between the wide-mouthed groups with teeth almost equally 

 developed on both sides of the jaws, and the small -mouthed group 

 with teeth more developed on the under sides of the jaws than on the 

 upper. Jordan cites a transitional form (32, p. 285), but none have 

 been seen by myself. Yet this character, for the reason stated above, 

 can only be regarded as of secondary importance in tracing affinities. 

 It is almost certain, for example, that the original primitive form was 

 wide-mouthed, akin to a Gadoid. Shall we say, then, that the Turbot, 

 which has retained this character but has altered in many others, is 

 more nearly allied to the primitive form than the Plaice, which has 

 altered in this character but retained the others ? Even if it were a 

 case of balancing the mere number of characters which had altered in 

 these groups, we should decide that the Plaice-group was more akin 

 to the primitive form, but the relative importance of the characters 

 weighs in the same direction. The mouth is not on the same footing 

 as, for example, the structure of the pectoral girdle, the position 

 of the anterior extremity of the dorsal fin, or the structure of the 

 olfactory organ. In the discussion of the various influences that 

 affect the characters, and of the relative importance of characters, 

 it will be shown how the Turbot-group is in reality more specialised 

 and distant than the Plaice-group from the Halibut and thence the 

 primitive stock. 



The relative positions of the various bones which form the boundaries 

 of the mouth — -premaxilla, maxilla, and mandible— as well as the 

 configuration of the bones which form the palatine and hyomandi- 

 bular arches, are not exactly the same in the different groups, but how 

 far the differences are generic, how far only specific adaptations, has not 

 yet been determined. This is also the case with the bones forming the 

 gill -covers and the hyoidean arch underneath. There are certain 

 differences present, but they are overbalanced by the general resem- 

 blances. Externally, the differences with regard to the opercular bones 

 are most marked in the Sole-group. In this the bones are thin and 

 slender, and their contours are completely hidden by the thick over- 

 lying skin, whilst in the other groups the skin is relatively not so thick, 

 and the contours are well marked. This difference, which can be 

 correlated to the more sand-loving habits of the Soles, has been used 

 by Jordan and other American writers to differentiate this group from the 

 others. It is not the sort of character that the morphologist would lay 

 stress upon, but its general nature makes it useful to the systematise 

 Accompanying it is another important characteristic of the Soles, 

 namely, the almost universal development of a " snout " anterior to the 

 mouth. This was employed by Giinther, and the two characters taken 

 together — signifying as they do a definite specialisation in habits — fitly 

 mark the separation and distinctiveness of the Sole-group from the 

 others. 



Certain other external differences between the various groups will be 

 considered under separate headings, because they accompany internal 

 differences in organs and structures. 



