SKULL. 



189 



The skull is always hyostylic and possesses both mem- 

 brane and cartilage bones. It differs considerably in the 

 extent to which the primitive cartilage persists. In 

 many forms, e.g. the salmon, pike (particularly in the less 

 specialised, more ganoid-like fishes), a considerable amount 

 of cartilage persists and the cartilage bones are separated 



Brs^ 



Fig. 110. — Cephalic skeleton of Perca fluviatilis (RSgne animal). Ac post-clavicles ; -.i/s 

 alisphenoid ; An angular ; Ar articular ; Brs branchio-stegal rays ; CI clavicle ; Car cora- 

 coid ; D dentary ; Ekp ectopterygoid ; Enp entopterygoid ; Ethi median ethmoid ; EtM 

 lateral ethmoid (prefrontal) ; Fr frontal ; Frp postfrontal (sphenotic) ; Hm hyomandibular ; 

 Hi, hyoid arch ; Jm premaxilla ; JOp intcroperculum ; Mtv metapterygoid ; Mx maxilla ; 

 Oex epiotic ; Op operculum ; Os supraoccipital ; Pal palatine ; Par parietal ; POp pre- 

 operculum ; PrO prootic ; Ps parasphcnoid ; Q quadrate ; S symplectic ; Sc scapula ; 

 SOp suboperculum ; Sq pterotic ; Ssc supraclavicle ; Vo vomer. 



by wide tracts of intervening cartilage. In others, e.g. the cod, 

 the cartilage is almost entirely ossified. The cartilage is usually 

 deficient in the roof of the skull except in the occipital region, 

 in which a basi-, two ex- and a supra-occipital are developed. 

 The auditory region usually presents five separate cartilage bones, 

 the epiotic, opisthotic, jarootic, the pterotic and the sphenotic 

 (postfrontal). The sphenoid region is feebly ossified : there 

 is always a small basisphenoid and sometimes an alisphenoid 



