PHYLUM CHORDATA 



203 



frontals, FR., and nasals, NA., and the unpaired supra-ethmoid, 

 S. ETH.) can be easily removed from the dorsal surface ; and two 

 unpaired bones (theparasphenoid.P^. SPH., and vomer, FO.) from 

 the ventral surface. These are all investing bones : they are 

 simply attached to the cranium by fibrous tissue, and can readily 

 be prised off when the latter is sufficiently softened by maceration 



or boiling. 



We thus get a distinction between the cranium as a 



sphpt 'par 





'pruop 



Fig. 859. — Salmo, the entire skull, from the left side. art. articular ; hrancliiost. branchio- 

 stegal rays ; dent, dentary ; epiot. epiotic ; eth. supi-aethmoid ; fr. frontal ; hyom. hyo- 

 mandibular; intop. interopercular ; Jug. jugal ; vipt. mesopterygoid ; mtipt. metaptery- 

 goid ; mx. maxilla ; nai. nasal ; o. suborbitals ; op. opercular ; pal. palatine ; par. parietal ; 

 pmx. prcmaxilla ; praop. preopercular ; pt. pterygoid ; pter. pterotic ; Quad, quadrate ; 

 sote. supraoccipital ; gphot. sphenotic ; subop. subopercular ; &ympl. symplectic ; Zunge, basi- 

 hyaL (From Wiedersheim's Vertebrata.) 



whole, or secondary cranium, complicated by the presence of invest- 

 ing bones, and the primary cranium or chondrocraniiom, left by 

 the removal of these bones and corresponding exactly with the 

 cranium of a Dog-fish. 



The primary cranium contains the same regions as that of 

 Scyllium. Posteriorly is the occipital region, surrounding the 

 foramen magnum, presenting below that aperture a single concave 

 occipital condyle for the first vertebra, and produced above into an 

 occipital crest. The auditory capsules project outwards from the 

 occipital region, and between them on the dorsal surface of the 



