THE SKULL. 



69 



Much of the cartilaginous primordial skull persists in most 

 Teleostei ; the cranial cavity, in all skulls described up to the 

 present time, may either reach between the eyes as far as the 

 ethmoidal region, or it may become narrowed and arrested in 

 the orbital region (Fig. 43, C). 



The palatoquadrate bar becomes differentiated into a perfect 

 row of bony plates, which are described as quadrate, meso- and 

 metapterygoid, pterygoid, and palatine. In the occipital and 

 auditory regions, as well as on the dorsal surface of the skull, 

 numerous groups of bone are developed, which cannot be further 

 described here. A canal, lying in the axis of the base of the skull of 

 many Teleosteans, must be mentioned : it encloses the eye-muscles, 

 and opens on each side into the orbits. 



Jlfl 



FIG. f>5. CRANIAL SKELETON OF TROUT. 



Ep, epiotic ; Ptr, pterotic ; Sph, sphenotic ; Os, supraoccipital ; P, parietal ; F, 

 frontal ; Sp.ctk, supra-etlimoid ; Can, aperture of the canal for the olfactory 

 nerve; Nl, nasal; Pmx, premaxilla ; M, M l , maxilla; /</, jngal ; Ms, ineso- 

 pterygoid ; Mtp, metapterygoid ; ooo, orbital ring ; Hm, hyomandibular ; s, 

 symplectic ; Qu, quadrate ; Pr, preoperculum ; lop, interoperculum ; Sop, sub- 

 opercuhim ; Op, operculum ; BsS, branchicstegal rays ; Ar, articular ; De, 

 dentary ; A, eye. 



All the bones bounding the oral cavity, viz., the vomer, the 

 parasphenoid, the premaxilla, and the maxilla, may bear teeth. 

 The maxilla, however, is usually edentulous, and both it and the 

 premaxilla vary much as to their development : the latter may 

 even be absent. 



Besides the above-mentioned palatoquadrate bar, the proper 

 cranial capsule of Teleosteans is surrounded by other outworks 

 consisting of plates and bars. These arise as dermal bones in the 

 region of the eyes (orbital ring) (Fig. 55, 000} , and in the gill- 

 covers (opercular bones) (Fig. 55, Pr, Op, Sop, lop}. A large 

 number of branchiostegal rays are developed in the ventral parts 

 of the opercular fold, or branchiostegal membrane (Fig. bo,JBsS). 



