XIII 



PHYLUM CHORDATA 



205 



The investing bones already referred to are clo.5ely applied to 

 the roof and floor of the chondrocranium, and modify its form 

 considerably by projectino- beyond the cartilaginous part, and con- 

 cealing apertures and cavities. The gveixtfrontols (FK) cover the 

 greater part of the roof of the skull, concealing the fontanelles, and 

 furnishing roofs to the orbits. Immediately behind the frontals is 

 a j)air of very small 2)(trictals {PA.) ; in front of them is an unpaired 



FR Jhn. SPH OT 



E BR4 



BHY 



8 BR.l 



Fig. SCO. — Salmo fario. Disarticulated skull with many of the investing bones removed. 

 The cartilaginous parts are dotted, foa. fontanelle ; /t. m. articular facet for hyomandiliular ; 

 ilck. C. Meckel's cartilage ; olf. .«. hollow for olfactory sac. Replacing bones — A.Xm. SPH. 

 alisphenoid ; ART. articular ; B. BR.l, first basibranchial ; B. HY. basiliyal ; B. OC. 

 basioccipital ; BR. 5, fifth branchial arch; B. SFK. basisphenoid ; C. BR.l, first 

 ceratobranchial ; C. HY. ceratoliyal ; EC. ETH.'ccto-cthmoid ; B. BR.l, first epi- 

 branchial ; E. HY. epi-hval ; EF.'OT. epiutic ; EX. OC. ex-occiiiital ; H. BR. 1, first 

 hyp..branclnal; H. HY. hypohyal ; HY. M. hyomandibular : I. HY. interhyal ; 

 MS. PTG. mesuiiterygoid ; MT. PTG. luctapterygoid ; OR. SPH. orbitosphenoid ; 

 PAIi. palatine; PH. BR.l. first pharyngoliranchial ; PTCr. pterygoid; PT. OT. 

 pterotic ; QIT. quadrate ; S. OC. supraoccipital ; SPH.OT. sphenotic ; SYHC. syiuplectic. 

 Investing bones — ANG. angular ; DST. dentary ; hR. frontal ; J U. jugal. ; MX. maxilla; 

 KA. nasal ; PA. palatine ; PA. SPH. [larasphenoid ; PMX. premaxilla ; VO. vomer. 



supra-dhmoid (S. ETH.), to the sides of which are attached a pair 

 of small nasals {NA). On the ventral surface is the large para- 

 sphenoid (PA. SPH.), which forms a kind of clamp to the whole 

 cartilaginous skull floor ; and in front of and below the parasphenoid 

 is the toothed vomer {VO). Encircling the orbit is a ring of scale- 

 like bones, the suhorhitals (Fig. 859, o.). 



In the jaws, as in the cranium, we may distinguish between 

 primary and secondary structures, The primary upper jaw or 



