114 LECTURE V. 



appendage to the supporting arch, is the pre-opemdar {fig. 30, 34), 

 which is usually the longest in the vertical direction, if not the 

 largest of the bones : it commonly presents a crescentic or an angular 

 form; it is sometimes bifurcate above, as in the Cod, and Avith the 

 lower slender angle continued downwards and forwards to beneath 

 the hypo-tympanic. In the Gurnards, or ' mailed-cheeked' Fishes, 

 the pre-opercular is articulated with the enormously developed sub- 

 orbital scale-bones. 



Tliree bones usually constitute the second series of this appendage : 

 the upper one is commonly the largest and of a triangular form, thin 

 and with radiated lines like a scale : it is the opercular {fig. 30. 35): 

 in the Cod it is principally connected with the posterior margin of 

 the pre-opercular, and below w^th the sub-opercnlar {ib. 36) ; but it 

 has usually, also, a "partial attachment to the outer angle of the epi- 

 tympanic, and is sometimes {Diodon, Lophius, Anguilla) exclusively 

 suspended therefrom. In the Lophius piscatorius the opercular is a 

 long and strong bone suspended vertically from the convex epi- 

 tympanic condyle, and with a long and slender fin-ray proceeding 

 from the back part of that joint. The sub-opercular forms the chief 

 part of the opercular fin by its long backwardly produced lower 

 angle. The sub-opercular bone in the Conger is soon reduced to a 

 mere ray, which curves backwards and upwards like one of the 

 branchiostegals. The opercular itself, though shorter and retaining 

 more of its laminated form, also shows plainly, by its length and 

 curvature in the Eels, its essential nature as a metamorphosed ray 

 of the tympanic fin. We have seen that all the framework of this 

 fin had the form of rays in the Plagiostomes. In Murfena the small 

 opercular bones articulate only to the imder half of the tympanic 

 pedicle. The sub-opercular is wanting in the Shad. The lowermost 

 bone, called the inter-opercular {fig. 30. 37) is articulated to the pre- 

 opercular above, to the sub-opercular behind, and usually to the back 

 part of the mandible ; it is attached, also, in the Cod by ligament to 

 the cerato-hyoid in front. The interopercular and preopercular 

 are the parts of the appendage which are most elongated in the 

 peculiarly lengthened head of the Fistularia. 



Hpoidean Arch {fig. 30. h, ii, 38 — 43). 



The third inverted arch of the skull is the ' hyoidean,' and is sus- 

 pended, in Osseous Fishes, through the medium of the epi-tympanic 

 bone to the mastoid ; and I regard it as the costal or htemal arch of 

 the parietal segment or vertebra of the skuU.* The first portion of 



* Bojanus, studying the vertebral homologies of the head in the fresh-water 

 tortoise, deemed the cornua of the hyoid to be the last costal arch of the skull, and 



