342 



Part III. — Eighteenth Annual Report 



they are attached by a strong ligament. The ventral fins are attached 

 to the bases of the laminar portions, and are directed posteriorly (Fig. 

 18). In the Citharus-group, we find a similar arrangement (Fig. 21), 

 whether the ventral fins are symmetrical — as in Paralichthys, or 

 asymmetrical — as in Ciiharichthys. When these fins are asymmetrical, the 

 laminar portion of the pubic bone of the upper or eyed side is attached 

 to the base of the clavicles (Fig. 21, v.l.). In the Sole-group when the 

 ventral fins are symmetrical, the clavicles are short and do not extend 

 to the ventral edge of the body. In this case the pubics are suspended 

 from near their base, and the ventral fins, though posterior to the 

 clavicles, have no direct connection thereto (Fig. 24). On the other 

 hand, the Turbot-group, as already noted in the " Scandinavian Fishes " 

 (p. 426), shows a remarkable difference from the above groups in this 

 respect. Beginning with Arnoglossus (Fig. 23), w 7 e find that the laminar 

 portion of the pubic bone of the eyed side is continued forward anteriorly 

 beyond the base of the clavicles by a cartilaginous plate (V.L). That of 

 the blind side has no cartilaginous plate, and stops short atafche base of 

 the clavicles (V.r.) In the Turbots this cartilaginous plate is present 

 on both pubic bones. These plates are closely and firmly approximated 

 to one another on their inner surfaces, and both extend beyond the base 

 of the clavicles as far as the anterior extremity of the lower prong of 

 the urohyal (Fig. 19, u.). According to the description in the work 

 cited, this cartilaginous plate is a part of the triangularly-shaped 

 laminar portion of the pubic. This is not exactly the case, however. 

 The cartilage is surrounded by an outer hard layer, but one can readily 

 detect its line of fusion with the osseous pubic (Fig. 19, c.V. and l.p.). 

 The rays of the ventral fins arise from the ventral edge of the cartila- 

 ginous plates, and are thus morphologically distinct from those of the 

 previous groups. These plates further have no attachment to the 

 clavicles, and depend solely from the pubics and urohyal.* 



The presence of these plates of cartilage affords a prolonged base of 

 attachment for the fin-rays, and we consequently find that in this case 

 the ventral fin-rays are more spread out and thicker than in the groups 

 where the cartilage is absent. 



When only one cartilaginous plate is developed, the ventral fin of that 

 side is prolonged anteriorly much in advance of that of the other side. 

 When both plates are developed, the rays of the one fin alternate with 

 those of the other fin, as in the Turbot. The fin which is anterior is 

 generally that of the upper or eyed side, but according to the American 

 writers the reverse is the case in Jlemirhom.bus (Syacium). 



These differences of internal structure in the various groups show 

 themselves in marked differences externally. In the Plaice- and Halibut- 

 groups, as also in the genus ISolea, the ventral fins are symmetrical with 

 regard to one another and to the ventral line of the body. In the 

 Turbot-group they are clearly asymmetrical. In Citharus, with its allies 

 Ciiharichthys and Hemirhombus, the ventral fins are also asymmetrical, 

 but it is necessary to note an important difference between this asym- 

 metry and that of the Turbot. In the former the cartilaginous plate is 

 absent and the fins are both behind the base of the clavicles (Fg. 21), in 

 the latter the reverse is the case. The ventral fins in these two groups 

 are therefore morphologically distinct in spite of the asymmetry being 

 present in both, and this important difference gives a means of forming 

 a new sub-family of the flat-fishes with Citharus as the type. 



* Cunningham (9) has proposed to call this bone jugular, because of its remote position 

 from that usually occupied by the urohyal. Although its form and position are not 

 the same, yet its relationships are similar — in that it is a median bone in line pos- 

 teriorly with the glossohyal, forming the isthmus between the branchial cavities and 

 connecting the pectoral arch with the hyoid bones. One is therefore justified in calling 

 it the urohyal. 



