PHYLUM OHORDATA 



205 



The investing bones already referred to are closely applied to 

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

 considerably by projecting beyond the cartilaginous part, and con- 

 cealing apertures and cavities. The gveaX frontals {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 pair of very small parietals {PA.) ; in front of them is an unpaired 



FRJhn, SPH.OT 



Fia. S60. — Salmo fario. Disarticulated -skull with many of the investing bones removed. 

 The cartilaginous parts are dotted. Son. fontanelle ; h. m. articular facet for hyomandibular ; 

 Mck. C Meckel's cartilage ; olf. s. hollow for olfactory sac. Replacing bones — AZi. SFH. 

 alisphenoid ; ART. articular ; B. BR. 1, first basibranchial ; B. HV. basihyal ; B. OC. 

 basioceipital ; BR. 5, fifth branchial arch; B. SFH. basisphenoid ; C. BB.l, first 

 eeratobranchial ; C. HT. ceratohyal ; EC. ETH.^ecto-ethmoid ; E. BR.l, first epi- 

 branchial ; E. K'V. epi-hyal ; EF.OT. epiotic ; EX. OC. ex-occipital ; H. BB. 1, first 

 hypobrauchial ; H. HV. hypohyal ; HV. IVE. hyomandibular ; I, IZV. interhyal ; 

 MS. FTC. mosopterygoid ; MT. FTG. metapterygoid ; OR. SFH. orbitosphenoid ; 

 FAIi. palatine; FH. BR.l, first pharyngobranchial ; FTG. pterygoid; FT. OT. 

 pterotic ; QU". quadrate ; S. OC. supraoccipital ; SFH.OT. sphenotic ; SVHS. symplectic. 

 Investing bones — ANG, angular ; I)NT. dentary ; T'R. frontal ; JU. jugal. ; MX. maxilla ; 

 NA. nasal ; PA, palatine ; PA. SPS. parasphenoid ; PMX, premaxilla ; VO. vomer. 



supra-ethmoid (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 



