XIII 



PHYLUM CHORDATA 



199 



considerably by projecting beyond the cartilaginous part, and con- 

 cealing apertures and cavities., x The great frontals (FR.) 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 

 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- 



RAL MS.PTG MT.PTG \ 



E SR4 



FIG. 871. Salmo fario. Disarticulated skull with many of the investing bones removed. 

 The cartilaginous parts are dotted, fon. fontanelle ; h. m. articular facet for hyomandi- 

 bular ; Mck. C. Meckel's cartilage ; olf. s. hollow for olfactory sac. Re placing bones 

 AL. SPH. alisphenoid; ART. articular; B. BR. 1, first basibranchial ; B. HY. 

 basihyal ; B. OC. basioccipital ; BR. 5, fifth branchial arch ; B. SPH. basisphenoid ; 

 C. BR. 1, first ceratobranchial ; C. HY. ceratohyal ; EC. ETH. ecto-ethmoid ; 

 E. BR. 1, first epibranchial ; E. HY. epi-hyal ; EP. OT. epiotic ; EX. OC. 

 ex-occipital ; H. BR. 1, first hypobranchial ; H. HY. hypohyal ; HY. M. hyomandi- 

 bular ; I. HY. interhyal ; BIS. PTG. mesopterygoid ; MT. PTG. metapterygoid ; 

 OR. SFH. orbitosphenoid ; PAL. palatine ; BR. 1, first pharyngobranchial ; 

 PTG. pterygoid ; PT.,OT. pterotic ; QTJ. quadrate ; S. OC. supraoccipital ; SPH. OT. 

 sphenotic ; SYM. symplectic. Investing bones ANG. angular ; DNT. dentary ; FR. 

 frontal ; JU. jugal; MX. maxilla ; NA. nasal ; PA. parietal; PA. SPH. parasphenoid ; 

 PMX. premaxilla ; VO. vomer. 



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 (70.). Encircling the orbit is a ring of scale- 

 like bones, the sub-orbitals (Fig. 870, o.). 



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

 primary and secondary structures. The primary upper jaw or 

 palatoquadrate is homologous with the upper jaw of the Dog-fish, 

 but instead of remaining cartilaginous, it is ossified by five replacing 



N 2 



