SKULL. 



189 



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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 



Os 



FlG. 110. — Cephalic skeleton of Perca fluviatUv) (R6gae animal). Ac post-clavicles; Ala 

 alisphenoid ; An angular ; Ar articular ; Brs branrhio-stegal rays ; CI clavicle ; Cor fora- 

 coid ; D rtentary ; Ekp erto pterygoid ; J?np entopterygoid ; Ethi median ethmoid ; EtU 

 lateral ethmoid (prefrontal) ; Fr frontal ; Frp postfroutal (sphenotic) ; Um hyomandibular ; 

 lit, hyoid arch ; Jm premaxilla ; JUp intcroperculum ; Mtp metapterygoid ; Mx maxilla ; 

 Oex epiotic ; Op operculum ; Os supraoccipital ; Pal palatine ; Par parietal ; POf> i)re- 

 operculum ; PrO prootic ; Ps parasphonoid ; 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-, tv/o ex- and a suiDra-occipital are developed. 

 The auditory region usually presents five separate cartilage bones, 

 the epiotic, opisthotic, prootic, the pterotic and the sphenotic 

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

 is always a small basisphenoid and sometimes an alisphenoid 



