Proceedings of the general meetings for scientific business of the Zoological Society of London, 1904 vol. 2

Page 97

74 DR. W. G. RIDEWOOD ON THE CRANIAL [May 3, grounds to be a primitive articulation persisting in this case, but lost in the great majority of Teleostean fishes. Such an articula­tion also exists in Arapaima and Heterotis. In the Mormyroid fishes the whole of the upper edge of the hyopalatine arch enters into extensive relation with the under surface of the cranium. The symplectic is usually rod-like, but in Arapaima, and to a less extent in Osteoglossum and Notopterus, it spreads in a squamous manner over the surrounding bones. It is wanting in the Mormyridae. The slope of the hyomandibular varies con­siderably in the different genera, as also does the angle which the symplectic makes with the axis of the hyomandibular. The ectopterygoid is in some cases straight (e. g. Hyodon), in others moderately bent (e. g. Clupea) or sharply angulate (e. g. Chatoessus). The hyomandibular articulates with the cranium by either one or two heads; and since two species of the same genus may vary in this respect, one may conclude that no great importance need be attached to the difference. The hyomandibular of Clupea jinta differs from that of Clupea harengus in possessing two separate heads for articulation with the cranium, whereas the latter has a single head, broad antero-posteriorly. Since Swin-nerton (I. c. pp. 541, 544, & 549) has shown that in the de­velopment of Gastrosteus the double head of the hyomandibular results from the enlargement of the anterior and posterior ends, and the reduction of the middle part, of a single broad head, Clupea harengus may possibly be more primitive than Clupea Jinta in this respect. One would imagine on general grounds that a diminution in the size of the mouth would be associated with a forward slope of the hyomandibular, a marked angulation between the symplectic and the hyomandibular, and a sharp bend in the middle of the ectopterygoid ; and this is certainly what one finds in Chatoessus, and to a lesser extent in Clupea harengus as compared with Clupea Jinta. Yet, while Megalops has a smaller mouth than Elops, its symplectic is more nearly in a line with the hyoman­dibular than in the latter genus; that is to say, the angle, as measured in degrees, is greater (c j text-figs. 14 & 10, pp. 46 & 40). The angulation of the ectopterygoid is approximately the same in both, and the advancement of the quadrate articulation in Megalops is brought about by the forward slope of the hyomandibular only. Hyodon has a fairly large mouth, associated with a perfectly straight ectopterygoid and a backwardly sloping hyomandibular, yet, as in Elops, the symplectic is strongly bent upon the hyomandibular. Somewhat similar relations-the symplectic angle, however, being more open-obtain in the large-mouthed genera Engraulis and Coilia. On the other hand, it is interesting to note that in the small-mouthed forms Gonorhynchus and Chanos the hyomandibular does not slope forward. In the latter genus the quadrate has shifted forward along the ectopterygoid, and has parted from the symplectic entirely.