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 articula- 

 tion 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 suiTounding bones. It is wanting in 

 the Mormyridse. The slope of the hyomandibular varies con- 

 siderably 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. Cliipea) or sharply angulate (e. g. 

 Chatoessids). 



The hyomandibular articulates with the cranium by either one 

 or two heads ; and since two species of the same gen vis may vary 

 in this respect, one may conclude that no great importance need 

 be attached to the difference. The hyomandibular of Clupea 

 finta differs from that of Clupea liarengxis in possessing two 

 separate heads for articulation with the cranium, whereas the 

 latter has a single head, broad antero-posteriorly. Since Swin- 

 nerton (Z. c. pp. 541, 544, & 549) has shown that in the de- 

 velopment of Gastrostetis the double head of the hyomandibular 

 resvilts from the enlai'gement of the anterior and posterior ends, 

 and the reduction of the middle part, of a single broad head, 

 Chipea harengus may possibly be m.ore 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 finta. Yet, while Megalops has a smaller mouth than 

 Elops, its symplectic is more nearly in a line with the hyoman- 

 dibular than in the latter genus ; that is to say, the angle, as 

 measured in degrees, is greater {cf. 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 Gonorhynchihs scnA Ghanos 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. 



