THE SKULL. 



65 



The olfactory sacs lie in the ventro-lateral parts of the nasal 

 region, which is often elongated to form a long cut-water or rostrum 

 (intertrabecula). Behind this are seen the deep orbital hollows 

 (Figs. 50 and 51), which are bounded posteriorly by the strongly 

 projecting auditory regions (GK). 



The palatoquadrate is usually only united to the basis cranii by ligaments, 

 but in the Chimaerae it becomes immovably fused with it, whence their name 

 of Holocephali. In some forms, the palatoquadrate is not directly united to 

 the skull, but is suspended from it by the upper segment of the hyoid arch or 

 hyomandibular (Fig. 51, Hm). In this case the skull may be described as 

 hyostylic, to distinguish it from -autostylic skulls, in which the hyoid 

 takes no part in the suspensorium. A cleft, the spiracle, lies on the 

 anterior border of the hyomandibular, and leads into the cavity of the mouth, 

 and on its walls may be found remnants of the embryonic spiracular 

 (mandibular) gill. 



The branchial skeleton is always richly developed, owing to 

 secondary segmentation and fusion of its parts, and exhibits 

 characteristic modifications. On the outer circumference of each 

 branchial arch radially-arranged cartilaginous rays are developed, 



FIG. 51. CRANIAL SKELETON OF Raja oxyrhyncha. 



GK, auditory capsule; Orb, orbit; NX, nasal capsule;'.??, rostrum; LK, LK\ 

 labial cartilages ; ^Sp, spiracular cartilage ; 8P l , spiracle ; PQ, palatoquadrate ; 

 Md, mandible; Hm, hyomandibular ; hy, hyoid ;'/ to V, first to fifth branchial 

 arches ; a, b, c, d, the individual segments of the branchial arches, viz. the pha- 

 ryngo-, epi-, cerato-, and hypohranchials ; t, point of union of the fourth and 

 fifth branchial arches ; Cp, basibranchials. 



which serve as supports for the gill-sacs. They are present also 

 on the hyomandibular and hyoid, and rudiments of mandibular 

 rays are present in Sharks. 



While in Elasmobranchs the gill-slits open freely on to the sur- 

 face of the body, in Chimserge a fold of skin arising from the 

 hinder border of the hyomandibular, lies over them. This is the 

 first indication of a gill-cover or operculurn, such as we shall 

 meet with again in Teleosteans and Ganoids. 



