964 



PISCES. 



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 by 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 (Jig. 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 



Fig. 497. 



Inferior view of the skull, branchial arches, and pec- 

 toral apparatus, of Squalus centrina. (After Cams.) 



a, nasal cavity; b, (.Ifactory organ; c, superior 

 labial or intermaxillary cartilage ; d, intermaxillary 

 bones ; e, inferior portion extending between the 

 preceding and the lower jaw; e, e, e, e, central 

 sternum-like pieces ; /. /, /,/, branchial arches; 

 ', i, i, branchial appendages ; k, scapular zone ; 

 I, m, n, pectoral fins. 



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 band 

 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 (jig. 497,J') consists of 

 two lateral halves united by a sympbysis ; each 

 half consists of a single piece of considerable 

 breadth, presenting a deep sulcus superiorly 

 for the lodgement of the teeth. 



The branchial apparatus is placed further 

 back than in the osseous Fishes, being situated 

 beneath the commencement 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 (Sturionida), which 

 seem in many respects to occupy an interme- 

 diate place between them and the osseous 

 division. 



The os hyoides in Squatina 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 Squatina there exists 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, while the second and third are 

 fixed to the sides of the central piece first men- 

 tioned. The arches supporting the branchiae 

 are five in number on each side, (Jig. 497, f\f) 

 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 

 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 framework is formed that almost entirely 

 surrounds the neck. The superior extremities 

 of these arches are fixed beneath the anterior 



