188 



ZOOLOGY 



SECT. 



certain portions of it are replaced by cartilage bones, and there are 

 in addition numerous membrane-bones developed in the surround- 

 ing connective tissue. As in the Dog-fish, the skull may be divided 

 into cranium, upper and lower jaws, with their suspensory apparatus, 

 and hyoid and branchial arches. 



The cranium (Fig. 812) is a somewhat wedge-shaped structure 

 its apex being directed forwards. At first sight the distinction 

 between cartilage and membrane-bones is not obvious, but after 



Sphot 



SOCb 



dent 



FIG. 811. Salmo fario, the entire skull, from the left side. <t,-t. articular; l>,-a,ichioist. 

 brauchiostegal rays; <h)it. dentary ; epiot. epiotic ; cth. supra-ethnoid ; TV. frontal: hyom. 

 hypmandibular ; intop. inter-opercular ; Jug. jugal; mpi. mesopterygoid ; mtpt. metaptery- 

 goid; mx. maxilla; nan. nasal; o. sub-orbitals ; o[>. opereular ; pnl. palatine ; JHI.: parietal; 

 pmx. pre-maxilla; praop. pre-opercular ; pt. pterygoid ; ptn: pterotic ; Q"<uL quadrate: 

 socc. supra-occipital; xpltot. sphenotic ; stibop. sub-opercular ; x//,// /( /. symplectic ; Z >.','< . basi- 

 hyal. (From Wiedersheim's Vertebrata.) 



maceration or boiling certain flat bones (the paired parietal*, PA., 

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

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

 unpaired bones (the parasphenoid, PA. SPff., and vomer, VO.) from 

 the ventral surface. These are all membrane-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 

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



