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Show 1904.] OSTEOLOGY OF THE ELOPID^E AND ALBULIDJG. 75 In Chirocentrus, although the mouth is fairly large, the quadrate is advanced by a forward rotation of the hyomandibular. The symplectic in this case is in a direct line with the axis of the hyomandibular. In Arapaima the mouth can hardly be described as small, yet the forward slope of the hyomandibular is excessive. The explanation of this is most probably to be sought in the great length of the postorbital portion of the head. In Engraulis and Coilia alone of the forms under consideration does the quadrate slope backward. Opercular Series.-The reasons for excluding the preopercular and interopercular bones from this series are given on p. 68. The bones considered under the present heading are the opercular .and subopercular bones, the branchiostegal rays and the jugular plate. As is well known, the subopercular bone is wanting in Notopterus. This condition is unparalleled among the other forms under consideration, although the subopercular is distinctly small in Osteoglossum and Arapaima, and very small in Heterotis. It is comparatively large in Albula and Gonorhynchus; it is said to be wanting in Pantodon. The lower Malacopterygian fishes are well adapted for demonstrating the continuity of the opercular and branchiostegal systems. The opercular and branchiostegal bones are functionally similar, serving to support the gill-cover, and there seems to be good reason for regarding them as morphologically similar also. The view is by no means a new one, for Traquair (" Ganoid Fishes Brit. Carb. Form.," Palseontogr. Soc.) demonstrated it in the case of the Palseoniscidse in 1877, and Shufeldt (Rep. U.S. Com. Fish. 1883 (85), pp. 818 & 820) mentioned it in 1883 ; and although the latter does not in his paper give references to previous expressions of such opinion, he does not himself claim the idea as original. The only recent reference to such a view appears to be that by Cole and Johnstone (Proc. & Trans. Liverp. Biol. Soc. xvi. 1902, p. 175), who mention the opercular bones (excepting the preopercular) as " modified branchiostegal rays." In Chanos, Albida, and Elops the transition from the uppermost branchiostegal rays to the subopercular and opercular bones is very evenly graduated, and in Osteoglossum it becomes a matter of some difficulty to decide whether the bone lying antero-ventrally to the opercular is a reduced and displaced subopercular, or the uppermost branchiostegal ray which has lost its connection with the epihyal. The difficulty of deciding whether a bone lying below the opercular bone is a subopercular or a free branchiostegal ray, the true subopercular being wanting, must frequently have occurred to the systematic ichthyologist. Boulenger distinctly admits the difficulty when dealing with the Mormyroid fishes (‘ Poissons du Bassin du Congo,' 1901, p. 50, footnote). In preparing the skull of Engraulis the subopercular comes away readily with the epihyal, and is very liable to be mistaken for a |