of the Fishery Board for Scotland. 



347 



Soles, in which the rays were only partially united. It was remarked 

 by him also that the appearance of the flat-fishes in the geological 

 records corresponded with these differences. The Turbot-group with 

 free supporting rays appears first in the Tertiary period, the 

 Solea next, whilst the Plaice-group is essentially modern. Such a 

 correspondence would be of great interest, suggesting, as was pointed out 

 by Sauvage, the possible lines of descent ; but it does not seem possible 

 to retain the distinctions made. All the species of the Turbot-group 

 which have been examined resembled those of the Plaice- and Halibut- 

 groups in having the hypurals as ossified plates and not composed of 

 separate rays. In some cases, indeed, e.g. the Turbot itself (Fig. 14), the 

 hypurals and epiural are fused into one piece, where ordinarily they are 

 divided into three (Fig. 13, ep., hy. 1 , hy. 2 ). When dried specimens are 

 examined, however, the ossification along the furrows shrinks 

 considerably, because the internal tissue is cartilaginous along these lines, 

 and the ridges become more prominent. The only explanation, there- 

 fore, of the facts described by Sauvage is that he has examined dried 

 specimens of the Turbot, and possibly spirit or fresh specimens of the 

 other species. These furrows contract within a few hours, if left to dry, 

 and also in spirit to a certain extent, so that it is advisable to make 

 comparisons with specimens which are under similar conditions. 



The bones composing the element supporting the caudal fin-rays are 

 made up chiefly by the three already mentioned, but two or even three 

 others may also take part. The neural and haemal spines of the second- 

 last vertebra aid invariably in the groups mentioned in supporting the 

 caudal fin-rays. Sometimes they seem to be furrowed like the hypurals, 

 but this has probably arisen from the fusion of a separate and secondary 

 ossification with the true spine, as shown in the drawings of Soles and 

 Turbot (Figs. 14 and 15). Sometimes the haemal — but not the neural — 

 spine of the third-last vertebra also develops so far posteriorly as to 

 support the first ray of the caudal fin on the ventral side. The extra 

 development of this spine over that of the corresponding dorsal makes 

 up for the greater relative development of the dorsal half of the fan- 

 shaped hypural, and makes the number of fin-rays equal on both sides 

 of the median line. 



The basal elements of the caudal fin are thus essentially the same in 

 the Plaice-, Halibut-, Citharus-, and Turbot-groups, and this might 

 have been expected beforehand from a survey of the other parts of 

 the skeleton. In comparison with the Soles, the skull and body- 

 skeleton of these groups are massive, and the movements and habits 

 of the animals consequently require a strong and powerful tail, 

 and thus a strong supporting structure. In the case of the Soles, the 

 skeleton is not so massive, nor do the animals grow to the same size as 

 in the other groups. We thus find in them a transition to a totally 

 different type of structure. In the true Soles (Fig. 15) there is little 

 apparent difference superficially from the Turbot and other groups. 

 The hypurals are marked by furrows and ridges, and the spines of the 

 second and third last vertebra? also take part in supporting the caudal 

 fin-rays. But the urostyle (Fig. 15, u.) remains short and distinguish- 

 able from the epiural, whereas in the previous forms it was indistinguish- 

 able. This decrease in size of the urostyle becomes greater in the more 

 specialised genera of the Sole-group, and is accompanied by a reduction 

 in the hypural bones, until in the most specialised, as in Animopleurops 

 (Fig. 17) and Gynoglossus, we obtain a semblance of the diphycercal 

 tail. How this change has taken place — whether the posterior vertebrae 

 in such as Ammopleurops (Fig. 17) are not really homologous to vertebrae, 

 but have arisen from modifications of the hypural elements, or whether the 

 x 



