As there is in the Chondropterygii no depo sition of bony particles radiating from ossific centres, there can be no division of the cranium into distinct bones, nor of course any sutures : the whole cranium consists of a single cartilagi nous piece, in which, however, it is easy to dis tinguish the same regions, the same fossw, the same eminences, and the sarne holes as in the skull of one of the osseous Fishes; but, although it is not difficult with a little attention to point out the situation of the different bones, to define their limits is impossible.
The bones of the face are likewise consoli dated with the great cranial mass, and conse quently are quite undistinguishable except from their position in relation to organs into the composition of which they enter.
The structure of the skull therefore appears exceedingly simple vvhen compared with that of an osseous fish. The whole pterygo-temporal apparatus is represented by two pieces, one of which corresponding with the temporal, tym panic, symplectic, and jugal bones of Cuvier, or the tympanic pedicle, as Professor Owen calls the long stem, which in the osseous Fishes is composed of those elements, is here represented by a single piece (fig. 496, c) in terposed between the side of the cranium and the point of junction between the upper and lower jaws—an arrangement precisely similar to that which is observable in the Batrachian Tteptiles.
The other piece belonging to the pterygo temporal apparatus ( fig. 496, e) forms, in con junction with its fellow of the opposite side, almost the whole of the upper jaw, covering the greater part of the roof of the mouth, and likewise supporting all the formidable teeth with which this jaw is armed. Posteriorly it gives attachment to the inferior maxilla hy two large articulating surfaces, and above it is only con nected to the skull by the muscles implanted into it. From its upper margin it gives off a process, shown in the figure, which remains permanently cartilaginous, and in the living state is imbedded among the muscles that form the inner wall of the orbit ; the whole is thus left completely moveable so as to give great latitude to the motions of the jaws.
The superior maxillary bone (fig. 497, d)and the intermaxillary bone (c) are of very small size, being merely imbedded in the substance of the upper lip and connected superiorly by ligament to the face and the piece last described. Inferiorly these bones are attached to a third (fig. 497, e,) which is fixed by ligaments to the outer surface of the lower jaw at about one-fourth part of its length from the symphysis, so that the three together form an osseous and ligamentous hand that circumscribes the angle of the mouth and materially diminishes the rictus of the jaws. The inferior piece (e) is most probably one of the elements belonging to the lower jaw detached from its usual connections with that bone.
The inferior maxilla (fig. 497, J) consists of two lateral halves united by a symphysis ; each half consists of a single piece of considerable breadth, presenting a deep sulcus superiorly for the lodgement of the teeth.
The bronchial apparatus is placed further back than in the osseous Fishes, being situated beneath the cominencement of the spine— a circumstance which causes the bones of the shoulder to recede backwards also.
The whole of the opercular apparatus is wanting in the cartilaginous Fishes with the exception of the Sturgeons ( Sturionide), which seern in many respects to occupy an interme diate place between them and the osseous division.
The os hyoides in Sguatina and the Sharks generally is composed of three pieces, one situated in the mesial line, and two lateral branches. The mesian piece or body of the os hyoides corresponds to the bones of osseous Fishes ; while the large rami, instead of mounting up to be connected with the sty loid bones, terminate immediately behind the articulation of the lower jaw, with which they are intimately connected by means of strong ligaments. From its posterior margin branchiostegous rays are given off precisely as in the former group, but these have nothing to do with the formation of an opercular flap, the branchial apertures being here of a very diffe rent character.
The branchial arches in their general arrange ment resemble those of the osseous Fishes, but there are nevertheless important differences to beremarked. In Sguatina thereexists inferiorly a kind of sternal apparatus which occupies the mesial line. This consists posteriorly of a cen-] tral piece (497, e, e,e,) that very nearly resembles a broad spear-head, forming a kind of sternum, the handle of the spear closely representing the xiphoid cartilage of the human sternum ; and in front of this are three pieces on each side, something like the costal cartilages. Of these the anterior pair are united to each other in the mesial line, vvhile the second and third are fixed to the sides of the central piece firstmen tioned. The arches supporting the branchim are five in number on each side, (fig. 497, /;.i) each consisting of an inferior and superior por tion connected with each other by moveable.. articulations. The inferior portion consists of a single piece, the superior of two, united together I by ligaments. The anterior arch is connected , by ligaments to the body of the os hyoides, and ' also to the central pieces. The four posterior are attached by ligament to the succeeding lateral processes of the sternal apparatus, and thus a fmmework is formed that almost entirely surrounds the neck. The superior extremities of these arches are fixed beneath the anterior vertebrze of the spine by a loose liganaento cellular substance.