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Show 210 ON THE STRUCTURE OF THE SKULL. cartilage, to the posterior part of. the o~ter surface of which is applied the bone (F, Fig. 84) with whwh the opercular bone ( Op) is moveably united by ligament. . . The bone (F) has, like most of the bones o~ the Lep~d~s~.ren, a green colour. Through the. greater p~r~ of Its length It IS so easily separated from the cartilage that It 1.s clearly a 1nembrane bone. Towards the C'ondyle, however, It adheres firmly to, thouah, on the application of a certain force, it springs away from~ a nodule of whitish bone, which lies in the very substance of the articular end of the cartilage, and repeats its pulley-like form. I suspect that this nodule, which represents the os quadrat·um, is primitively distinct . from the bone (F). The latter, under these circumstances, would have much analogy with the pre-operculum of osseous fishes, and Op would cmTrspond with the sub- or inter-operculum. All other fishes, comprising such Ganoidei as have not been already mentioned, and the Teleostei, have, so far as is at present known, the palata-quadrate arch primitively distinct from the hyomandibular suspensor; the latter is, priinitively, moveable upon the skull; and, in the walls of the cranium, the pro-otic bones, at least, are ossified as well as the ex-occipitals; that is to say, they are constructed essentially upon the plan of the Pike. The modifications they exhibit in detail are almost infinite, but a few of the most important n1ay be enumerated :- 1. The cartilaginous cranium persists throughout life in such fishes as the Pike and the Salmon; in very many, as the Perch and the Carp, it disappears almost entirely. 2. In most fishes the basis cranii is compressed from side to side in the orbital region, and vertically enlarged, so as to form an inter-orbital septum, which, ns it were, encroaches upon the cranial cavity and narrows it anteriorly. But in others-such as the Cyprinoids and the Siluroids-no inter~orbital septum is developed, the basis cranii ren1aining flat, and the cranial cavity of nearly equal size throughout. 3. The last-n1entioned fishes have the cranial walls completely occupied by bone, distinct ossifications representing the alisphenoids and orbito-sphenoids. THE SKULLS OF FISHES AND AMPHIBIA. 211 . 4. ~he _opisthotic bone, occasionally absent as a distinct ossificatiOn, Is very small in some fishes such as the p h ( h · · 0 · ' ' ere w ere It IS uvi~r s "rocher" or "petrosal"), but becomes very well d.e ve.l oped In such genera as EphirJrrrJU S ' and atta1"ns ca n I· mmense size In the Gadidm. 5. The canal for the orbital muscles is absent in man fishes such as the Cod tribe. y ' 6. The most r.e markable modification of the fi sh 's crani· um proper, however, Is t~e want of symmetry produced in the flat fishes~ or Pleuronect~dm, by a sort of twist, which affects the antenor and ~pper, but not the hinder and inferior, part of the sku·l l.· 1T hu· s, If the skull of a Turbot be examined , th e supra-ocCipita will be found in its ordinary place; while the epiotics and squamosals are symmetrically disposed on each side f · t tha~ the skull, vie~ed from behind, is like that of an; ~th:~ ordrnary os~eou~ fish. The basi-occipital, parasphenoid, and vo~er are likewise arranged, as usual, along the median basal axis of t~e skull. The p~o-otics .and post-frontals are also nearly s!mmetncal, but the ahspheno1ds are thrown over to the left side, so that the anterior aperture of the cranial cavit b _ tween the . alisphenoids, lies no longer immediate! y ov[.; t:e parasphenmd, but to the left of it. The left frontal sends down a long curve~ process, which joins with one from the prefrontal of t?e same side, and the two eyes come to lie in the secondary orbit, develo~ed between the curved bony boundary thus formed and the median frontal crest. 7 · ~n addition takes place to the posterior extremity of the skull, m many fishes, by the anchylosis with it., and with one another, of· a variable number of vertebne. Cartilaginous vert - b~oo, as I ha~e already pointed out, coalesce with the cartil:~~: ous skull In both Accipenser and Spatularia, and two or three y vertebroo are anchy losed with the osseous skull in L ·a teus d p z t eptt os-th a 1 n ~ ~p erus. _Whether a similar addition takes place in S "el ot .1d e r living gano d A · I ' m~a, or not, I cannot say In man v \~roi s a great number of vertebroo become thus . anchylosed WI one another and with the skull. f8. Jnhboth Siluroids and Ganoids, again, an addition to the _roo o t e skull is effected by the coalescence therew~th of the p 2 |
