12 



VERTEBRATA 



PHYLUM VIII 



occipital), which belongs to the cranial roof, often thrust between the parietal 

 bones, and provided with a vertical median crest. 



The auditory capsules are surrounded laterally by several pairs of bones 

 often completely covered by the facial and other membrane bones. Among 

 them are distinguished immediately in front of the exoccipitals the opisthotic 

 (OpO), the epiotic (EpO), adjoining the latter in its hinder upper part, and 

 the prootic (PrO), which is usually pierced by the trigeminal nerve. With 

 these bones, which form the auditory capsule proper, there is connected a 

 bony plate on each side above and in front of the opisthotic, which originates 

 partly from membrane, partly from cartilage, and is named the squamosal 

 (Sq) or pterotic (W. K. Parker) ; the mandibular suspensorium (hyoman- 

 dibular) articulates with its lower face. 



The lateral investment of the middle orbital region of the skull, which 

 sometimes remains cartilaginous or membranous, and is sometimes more or 

 less completely ossified, consists externally of facial bones. Two pairs of 

 bones can be distinguished here, posteriorly the alisphenoid placed just in 

 front of the prootic, and anteriorly the orbitosphenoid ; both pairs, but 

 especially the orbitosphenoids, often fuse together completely in the middle 



line. The base of the skull 

 A Ec P"- s.o is formed by a large dermal 



bone, the long, sword- 

 shaped parasphenoid, 

 which abuts behind on the 

 basioccipital, in front on 

 the vomer. 



The roof of the two 

 hinder regions of the skull 

 only rarely arises in carti- 

 lage ; it usually consists 

 of membrane bones, which 

 are intimately connected 

 with the cartilage ele- 

 ments. There are thus 

 intercalated between the 

 supraoccipital and the 

 squamosals two parietal 

 bones, and these are 

 followed in front by the 

 two frontal bones, which 

 often fuse together into a 

 large simple plate. By 

 the side of the latter and 



/ Ell, 



Fig. 24. 



Cranial skeleton of the Salmon. The persistent cartilage of the 

 cranium dotted ; the bones developed from cartilage lettered in italics, 

 the membrane bones in Roman type. A, Eye; Ang, Angular; Art, 

 Articular; Br.R, Branchiostegal rays; C.lly, Ceratohyal; D, Dentary ; 

 E.Hy, Epihyal; Ep.O, Epiotic; Eth, Ethmoid; Fr, Frontal; G.Hy, Glosso- 

 h yal ; H.Hy, Hypohyal ; HM, Hyomandibular ; IOp, Interoperculum ; Ju, 

 Jugal ; L, Lachrymal; M.l't, Mctapterygoid ; Ms. Ft, Mesopterygoid ; Mx, 

 Maxilla; Na, Nasal; Op, Operculum; Pa, Parietal; Pmx, Premaxilla ; 

 POp Preoperculum ; Pt, Palatine] PtM Pterotic (mth squamosal) ; Qu, ' m f ront f the squamosal 

 Quadrate; S.O, Supraoccipital; Sb. Or, Suborbitals; SOp, Suboperculum ; 1 «■ i 



S< )r, Supraorbital ; Sy, Symplectie (after Parker). there OCCUrS the postirontal 



(sphenotic, W. K. Parker), 

 which belongs to the membrane series in the Ganoids, but is a cartilage bone 

 in the Teleosteans. There is also occasionally a small supraorbital above the 

 eyes. 



The anterior ethmoidal region possesses as its base the vomer (/""<>) or 

 ploughshare bone, which also forms the anterior part of the roof of the hard 



