XITI 



PHYLUM CHORDATA 



205 



The investing bones already referred to are closely applied to 

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

 considerably by projecting beyond the cartilaginous part, and con- 

 cealing apertures and cavities. The great frontals (FU.) cover the 

 greater part of the roof of the skull, concealing the f bntanelles, 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 





E8R4 



Fio. 860. Salxno fario. Disarticulated -skull with many of the investing bones removed. 

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

 Mck. C. Meckel's cartilage ; off. s. hollow for olfactory sac. Replacing bones AL. SPH. 

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

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

 ceratobranchial ; C. HY. ceratohyal ; EC. ETH.-ecto-ethmoid; E. BR.l, first epi- 

 branchial; E. HY. epi-hyal ; EP.OT. epiotic ; EX. OC. ex-occipital ; H. BR. 1, first 

 hyp( > branchial ; H. HY. hypohyal; HY. M. hyomandibular; I. HY. interhyal ; 

 MS. PTG. mesopterygoid ; MT. PTG. metapterygoid ; OR. SPH. orbitosphenoid ; 

 PAL. palatine; PH. BR.l, first pharyngobranchial ; PTG. pterygoid ; PT. OT. 



terotic ; QU. quadrate ; S. OC. supraoccipital ; 8FH.OT. sphenotic ; SYIK. symplectic. 

 nvesting bones ANG. angular ; DNT. dentary ; PR. frontal ; JU. jugal. ; MX. maxilla ; 

 NA. nasal ; PA. palatine ; PA. SPH. 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 suborbitals (Fig. 859, o).* 



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

 primary and secondary structures. The primary upper -jaw or 



