88 COMPARATIVE ANATOMY 



rostrum, at the proximal end of which the olfactory sacs arc 

 situated, their cavities being separated from the cranial cavity by a 

 membrane. Behind them are the deep orbital hollows, which are 

 bounded posteriorly by the strongly projecting auditory capsules. 

 Labial cartilages (cf. p. 82) are present in connection with the lips, 

 nostrils, and jaws. 



The palatoquadrate meets with its fellow in the middle line and 

 is usually connected with the basis cranii by ligaments (Fig. 65). A 

 process may be present on it which articulates at some point with 

 the trabecular region. In the Chimseroids (Fig. 66) it becomes 

 immovably fused with the cranium, whence their name of 

 Holocephali. In the Sharks and Rays the palatoquadrate is not 

 directly united to the skull, but is suspended from it by the 

 hyomandibular (p. 82, and Fig. 65). 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. In Notidanus, 

 both mandibular and hyoid arches are independently connected 

 with the skull, which is therefore spoken of as ampliistylic. A 

 vestigial cleft, the spiracle, is situated in front of the hyomandi- 

 bular, and leads into the pharynx ; on its anterior wall may be 

 found remnants of the embryonic spiracular gill, beneath which 

 are one or more spiracular cartilages which probably represent gill- 

 rays (cf. below). 1 



In Plagiostomes the palatoquadrate and lower jaw are provided 

 with numerous teeth, arranged in rows ; in the Holocephali the 

 teeth have the form of strong and sharp-edged plates. 



The branchial skeleton is relatively smaller in the Holocephali 

 than in other Elasmobranchs, in which it is always richly developed, 

 and owing to secondary segmentation and also to fusion of its 

 parts, exhibits characteristic modifications. On the outer circum- 

 ference of each branchial arch, as well as on 'the hyomandibular 

 and hyoid, radially-arranged cartilaginous rays are developed, which 

 serve as supports for the gill-sacs (Fig. 65). Externally to these 

 rays rod-like " extra-branchial " cartilages are present : these 

 correspond to the displaced uppermost and lowermost gill-rays. 



In Plagiostomes the gill-slits open freely on to the surface of 

 the body, but in the Holocephali a fold of skin, the gill-cover or 

 operculum arising from the hinder border of the hyomandibular, 

 overlies them. In the frilled Shark (Chlamydoselachus) there is 

 an indication of an operculum. 



Amongst Ganoids, the lowest condition is met with in 

 those forms in which the hyaline primordial skull is still retained, 

 immovably fixed to the vertebral axis, part of which becomes 

 secondarily assimilated to it. These forms are spoken of as 

 Cartilaginous Ganoids. The presence of definite bones, however, 



1 A small basimandibular element has been described in Lsemargus, and 

 mandibular rays can be recognised in the primitive Pleuracarithidte from the 

 Permian formation. 



