THE SKULL 



75 



strongly projecting auditory regions. Labial cartilages are present 

 in connection with the lips, nostrils, and jaws (Figs. 55, 56, and 57). 



The palatoquadrate is usually only united to the basis cranii by 

 ligaments, but in the Chimseroids (Fig. 57) it becomes immovably 

 fused with it, whence their name of Holocephali. In the Sharks 

 ^nd Rays the palatoquadrate is not directly united to the skull, but 

 is suspended from it by the hyomandibular (p. 70, Figs. 55 and 56). 

 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 (Fig. 57). A cleft, the spiracle, is situated in 

 front of the hyomandibular, and leads into the cavity of the mouth ; 

 on its anterior wall may be found remnants of the embryonic 

 spiracular gill, beneath which is a spiraeular cartilage (comp. p. 70, 

 and Fig. 56). 



The branchial skeleton is always well developed, and owing 

 to secondary segmentation and fusion of its parts exhibits char- 



Fig. 56. — Skull of Skate. (After W. K. Parker.) 



Au, auditory capsule ; Na, olfactory capsule ; P.N, prenasal rostrum ; Pl.Pt, Qn, 

 palatoquadrate bar ; Mch, mandibular (Meckel's) cartilage ; 21. Pt, spiracular 

 cartilage ; H.M, hyomandibular ; i.h.l, interhyal ligament ; E.Hy, epihyal ; 

 C.Hy, ceratohyal; H.Hy, hypohyal ; H.Br, 1, 2, 5, hypobranchials ; above 

 them are seen the cerato-, epi-, and pharyngo-branchials ; //, optic foramen ; 

 V, foramen for trigeminal and facial nerves. (The branchial rays and extra- 

 branchials are not indicated. ) 



acteristic modifications. On the outer circumference of each 

 branchial arch, as well as on the hyomandibular and hyoid, radially- 

 -arranged cartilaginous rays are situated, which serve as supports 

 for the gill-sacs (Fig. 55). Externally to these rays small rod-like 

 " extra-branchial " cartilages are present. 



In nearly all Elasmobranchs the gill-slits open freely on to 

 the surface of the body, but in Chlamydoselache and the Holo- 

 cephali a fold of skin arising from the hinder border of the 

 hyomandibular overlies them. This is the first indication of a gill- 

 cover or (yperculum, such as is present in Teleosts and Ganoids. 



Amongst the Ganoids, the lowest condition is met with in 



