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Part III. — 'Eighteenth Annual Report 



As an additional protection to the kidney in Citharus, we find that the 

 primary ribs of the abdominal vertebrae anterior to those bearing 

 hcemapophyses are prolonged greatly posteriorly and ventrally over the 

 hcemapophyses and enclose the kidney. 



This character very evidently marks a specialisation in structure and 

 habits and cannot be used in classification. Such is the case also with 

 the pyloric caeca. As mentioned by Sagemehl, these vary in a strange 

 and at present unaccounted-for manner even in such as the Clupeidae 

 and Salmonidse. In the Heterosomata four seems to be the maximum 

 number, but there may be none at all. In the Plaice-group, two, three, 

 or four may be present ; four are found in the Halibut, Rhomboidichthys, 

 and Pseud orhombus; whilst none are found in Psettodes, Rhombosolea, and 

 possibly many more. P. cynoglossus, on the other hand, is similar to the 

 Plaice in this respect. The only group in which this character seems to 

 be constant is that of the Soles, and here there are none at all. As 

 these caaca are in most cases small it is sometimes difficult to say 

 whether slight protuberances or swellings on the intestine are worthy 

 to be called caaca or not, and this occurs in Citharichthys, Parophrys, 

 Br adiy pleura, and possibly many more. These seem to represent the 

 transition-stages between the presence and absence of the caaca. 



Hypural Elements. 



The terms by which the bones of the posterior region of the vertebral 

 column are described have been fixed by Huxley (25). The broad fan- 

 shaped bone continuous with the vertebral column is the hypural bone 

 (hy. 1 , Figs, 13-16). Dorsal and ventral to this are two bones which are 

 very similar in appearance, but very dissimilar in their relation to other 

 structures. The dorsal wedge-shaped bone has developed in connection 

 with the urostyle, the ossification which supersedes the termination of the 

 notochord, and is dorsal to it, as is shown clearly in the case of the Sole 

 (Fig. 16). Hence this should be called the epiural, according to 

 Huxley's terminology. The ventral wedge-shaped bone has no such 

 relations, however, and is not dignified by any other name than hypural, 

 but in order to distinguish it from the above it may be called the second 

 hypural (hy. 2 ). The relationships of these bones to one another, and to 

 the spines of the second-last vertebra, are not always the same in the 

 various groups of flat-fishes. The fan-shaped hypural bone in the adult 

 is divided into two portions which may be quite separate from one 

 another, but are as a rule joined by a thin connecting layer of bone. 

 The mark of division is not quite in the median line, but inclines 

 slightly downwards, so that the ventral portion is smaller than the 

 dorsal. The distal end is marked by furrows and ridges — the latter 

 being in line with the caudal fin-rays, whilst the furrows pass backward 

 towards the centrum of the last vertebra to a varying extent, some 

 going almost the whole length, others only a little way. The furrows 

 are ossified superficially only, the internal structiu-e being cartilaginous, 

 so that a large amount of flexure is possible. The second hypural and 

 the epiural are similarly furrowed. 



The number of ridges and furrows varies, of course, in the different 

 species, but if only the Plaice-, Halibut-, Citharus-, and Turbot-groups be 

 considered, it will be seen that there is no essential difference between 

 them. According to Sauvage (47), three types or three forms of hypural 

 structures were to be found in the flat-fishes — the one, Plaice- or 

 Pleuronectid-type, in which the hypural bones were completely ossified ; 

 another, Turbot-type, in which the hypural bones were divided into 

 separate rays to support the caudal fin-rays ; and a third type, in the 



