214 



ICHTHYOLOGY. 



Introduc- hj'po-tympanic, reaching almost to the mandibular trochlea. 

 "°°- It is the " symplectic" of Cuvier, and it is confluent in the 

 Osteology. giij,yoi(i, Muraenoid, and some otlier fishes with the epi- 

 ^^''^^ tympanic. The pre-tympanic "caisse"or "os tympanicum" 

 of Cuvier, receives, in its posterior margin, the fore part of 

 the meso-tympanic, and the upper and fore part of the 

 hypo-tympanic. It is confluent with the latter in Conger 

 and Mura:na, and it does not join either of these bones in 

 the Lepidosteus. Tlie hypo-tympanic bears the convex 

 articular trochlea for the lower jaw upon its inferior apex. 

 The Epihulus presents a remarkable, much elongated, 

 and slender modification of it, carrying the lower jaw at 

 an unusual distance from the base of the skull, and al- 

 lowing it to swing backwards and forwards on its long 

 pedicle as on a pendulum. In the Murmnidce the tympanic 

 pedicle is a strong triangular plate aflixed to the cranium 

 by its base or shorter side, and carrying the mandibular 

 joint backwards beyond the occiput. The mandible or 

 lower jaw is the haemapophysis of the frontal vertebra, and 

 completes the arch below by its ligamentous or bony sym- 

 physis with its fellow. Each branch or half consists most 

 commonly of two pieces, sometimes of three or more, 

 the one articulated to the suspensary pedicle being the 

 proper haemapophysis, and the extreme one completing 

 the arch and supporting the teeth being the haemal spine. 

 The diverffing apparatus of the tympano-mandibular arch 

 consists of the opercular bones of the gill-cover, which, by 

 its movements, opens and closes the branchial aperture on 

 each side of the head, thereby regulating the currents 

 through the branchial cavity. It may be considered as a 

 kind of short and broad fin. The first or most anterior, 

 and the chief medium of attachment of the appendage, is 

 the pre-opercular, which commonly has a crescentic or 

 angular form. In the Cod it is forked above, and in the 

 Sclerogenida the enormously developed second suborbital 

 scale bone crosses the cheek to be articulated to it. The 

 opercular is commonly the largest of the remaining pieces 

 of this appendage, and is chiefly connected with the poste- 

 rior margin of the pre-opercular, but very generally it is 

 partially attached above to the outer angle of the epi-tym- 



a long slender fin ray, and the sub-opercular, which forms introduc- 

 the posterior and lower angle of the gill-cover, carries out tion. 

 the resemblance to a fin by its radiated structure. All the Osteology, 

 framework of this fin has the form of rays in the Plagio- '^-"•■v^"' 

 stomes. The lowermost piece, called the inter-opercular, is 

 articulated to the pre-opercular above, the sub-opercular 

 and angle of the opercular behind, and usually to the back 

 parts of the mandible in front. In conjunction with the 

 pre-opercular, it is more elongated in the very long head 

 of Fistularia than the other two pieces of the appendage. 

 It is wanting in the Siliiri, and the sub-opercular is wanting 

 in the Shad. In Murcena, the small opercular bones are 

 articulated only to the lower part of the tympanic pedicle. 

 The smoothness or serratures of the posterior edges of these 

 bones, more especially of the pre-opercular, and the num- 

 bers and situations of the spines which emanate from them, 

 furnish characters to ichthyologists for the discrimination of 

 many species and groups of species. 



2. The hyoidean arch is suspended in osseous fishes, as 

 we have already had occasion to say, to the mastoid, through 

 the medium of a fork of the epi-tympanic bone ; and its 

 bony arch is completed by the two small, cubical hasi- 

 hyals. The anterior part of the arch is formed by the stylo- 

 hyal, epi-hyal, and cerato-hyal, the first-named being joined 

 at its upper end by ligament to the epi-tympanic, and the 

 others descending from it in succession. The cerato-hyal 

 is the longest and largest, and supports the diverging ap- 

 pendix of the hyoidean arch named brancliiostegals or gill- 

 cover rays, and on which the membrane that closes the 

 branchial chamber is extended. The number of these rays 

 varies greatly. They are only three, and these comparatively 

 broad ixnA flat in the Carps, while in Elops saurus they 

 amount to more than thirty. They are of very great length 

 in the Angler, and spread out the gill membrane like a great 

 bag on each side of the head. 



Fig. 20. 

 Hyoid bone of the Perch. 



panic. In the Lophius, its posterior margin is coasted by 



Fig. 21. 

 Gills of the Perch. 



In Pisoodonopliis cancrivorus the brancliiostegals are 

 thirty-three in number on each side, very long, and as fine 

 as hairs, and the curves they make in the branchiostegal 

 membrane are different on the two sides of the head. 



d. The branchial arches, which appertain to the system 

 of the visceral skeleton, succeed tiie hyoidean arch, and are 

 more or less closely connected with its key-stone, the basi- 

 hyals. Six of these arches are usuallv developed in em- 

 bryo, and five are usually permanent, four of them supporting 

 the gills, and the fifth circumscribing the opening of the 

 gullet and beset with teeth. This last is called the pharyn- 



