of the Fishery Board for Scotland. 



345 



haemal spine is strong and well developed in all these species, and the 

 appearance of the outer point externally cannot be relied upon. 



In the Oitharus- and Turbot-groups the first interhasmal spine is also 

 large and well developed (Fig. 8), but in Rhomboidichthys it extends only 

 half-way along the hasmal spine, and in Citharus but a little further. 

 The outer end of the interhasmal spine, however, lies some distance 

 within the skin, and is continued forward by partly ossified plates of 

 cartilage (Fig. 8, c), from the ventral edge of which the interspinous 

 rays arise which support the anterior rays of the anal fin. There can 

 be no chance, therefore, of an " anal spine " appearing externally. In 

 the Sole-group (Fig. 9, i.s.) the first interhasmal spine is much reduced, 

 and arises from near the end of the first haemal spine. In place of the 

 one interhasmal spine of the previous groups, in the Sole-group there 

 are four or five. These are not prominent externally, and thus present 

 no anal spine. 



The remaining groups of the flat-fishes, those of the Australasian fauna, 

 have not yet been so carefully examined with regard to these characters. 

 In Rhombosolea and Brachypleura the arrangement of the various struc- 

 tures is the same as in the Sole-group ; the last four to six abdominal 

 vertebras bear hasmapophyses, and the first interhasmal spine is slender 

 and little developed. It is most probable that this arrangement is the 

 same in the other forms, Ammotretis and Peltorham.phus, because the 

 external configuration of the abdominal cavity is similar. 



It appears, therefore, that the presence of haemapophyses on the 

 abdominal vertebras is a valuable character for displaying affinities. 

 These are found universally amongst the Soleidas, and in certain genera 

 of neighbouring groups. From their presence in the latter we may 

 conclude that these genera have a certain relation to the Soleidas closer 

 than that possessed by the remaining genera of the same groups. 



An example has already been given of the utility of this for purposes 

 of classification. When more genera of the Citharus-group have been 

 examined, this character may again be of use in aiding to subdivide this 

 group. It also aids in differentiating the group of flat-fishes in the 

 Australasian fauna from the Plaice-, Halibut-, and Turbot-groups within 

 which they were formerly placed. 



With regard to the alimentary canal, the differences found in the 

 various groups are connected with two characters : the intestine — its 

 increase or decrease in length, and the pyloric ca^ca — their presence or 

 absence. The increase in length of the intestine is chiefly well-marked 

 in the Sole-group and in one or two forms which in some way resemble 

 the Soles, e.g. the Lemon-sole (P. microcephalics ). In these, two to four- 

 coils of the intestine lie along the interhasmal spines of the eyed side, 

 within the secondary body-cavity, which contains also the reproductive 

 organ of that side. This occurs in many other forms, e.g. Rhomboidichthys, 

 Plaice, etc., but not to the same extent, the intestine merely projecting 

 into the second body-cavity. It may be that the projection of the 

 intestine into the latter cavity has the advantage of protecting the 

 kidney from injury. In the Sole-group, for example, owing to the 

 presence of ha'inapophyses on the posterior abdominal vertebras, the 

 primary abdominal cavity is very small, and some compensation for 

 this is therefore necessary. In the Lemon-sole, although hmmapophyses 

 are not present on the abdominal vertebras, the increase in length of the 

 intestine alone has rendered it necessary apparently to have more room 

 than the primary abdominal cavity can give. On the other hand, this 

 compensation may be obtained by the reduction in length of the intestine. 

 Such is found in Citharus, for example, where the posterior abdominal 

 vertebrae bear haimapophyses, but where the intestine is very short. 



