XIII 



PHYLUM CHORDATA 



193 



S.0r 



Pa.ch. 



JIM 



G.//I/ JU[y Mck Sy 



FIG. 813. Skull of young Salmon, second week after hatching ;. 

 the membrane bones removed. Au. aviditory capsule ; Br. 1, 

 first branchial arch ; Ch. notochord ; C. Hy. hyoid cornu ; 

 Fo. fontanelle ; G. Hy. basi-hyal ; H. Hy. hypo-hyal ; H. M. 

 hyomandibular ; I. Hy. inter-hyal ; /I, /2 ; labial cartilages; 

 Mck. Meckel's cartilage ; M. Pt. meta-pterygoid region of 

 primary upper jaw ; Pa. ch. parachordal ; PL Pt. palato- 

 pterygoid region ; Qu. quadrate region ; S.Or. supra-orbital 

 region of cranium ; Sy. symplectic region of suspensorium ; 

 T. Cr. cranial roof ; Tr. trabecula ; //, optic foramen ; V y 

 trigeminal foramen. (From Parker and Bettany's Morphology 

 of the Skull.) 



latter, at about the second week after hatching, the only ossifica- 

 tions present are 



a few membrane 



bones ; when these 



are removed we get 



a purely cartilagin- 

 ous skull (Fig. 813), 



exactly comparable 



with that of an Elas- 



mobranch. There is 



a cranium devoid of 



cartilage bones and 



divisible only into 



regions; the upper 



jaw is an unossined 



palato-quadrate (PI. 



Pt., M. Pt., Qu.) and 



the lower jaw (Mck.) 



a large Meckel's 



cartilage ; the sus- 

 pensorium is an 



undivided hyo-man- 



dibular (HM.), and the hyoid and branchial arches are unsegmented. 

 The first dorsal and the ventral fins are supported each by a triple 

 set of pterygiophores, so that the fin-skeleton 

 is multiserial, as in the Dog-fish. The proxi- 

 mal series consists of slender bony rays the 

 interspinous bones (Fig. 817, PTG. ; Fig. 814, 

 PTG.l), lying in the median plane, between 

 the muscles of the right and left sides, and 

 more numerous than the myomeres of the 

 regions in which they occur. Their distal 

 ends are broadened, and with them are con- 

 nected the second series (PTG. 2) in the form 

 of small dice-box shaped bones; to these, 

 finally, are attached small nodules of cartilage 

 (ptg.S) forming the third series of radials. 

 The dermal fin-rays (D.F.R.), which lie in 

 the substance of the fin itself, are slender 

 bones, jointed like the antenna of an Arthro- 

 pod, and mostly branched in the sagittal 

 plane (Fig. 817, D.F.R.). Each is formed of 

 distinct right and left pieces (Fig. 814), in 

 close contact for the most part, but diverging 

 below to form a forked and dilated end, 

 which fits over one of the cartilaginous 

 nodules (ptg.3). In the caudal fin (Fig. 810) 



VOL. II ' O 



&F.R 



PTG.1 H 



FIG. 814. Salmo farip. 



A dermal fin-ray with its 

 supports. D.F.R. dermal 

 fin-ray ; PTG.l, proximal 

 pterygiophore (inter-spin - 

 ous bone); PTG 2, middle 

 pterygiophore ; ptg.3, dis- 

 tal pterygiophore (cartila- 

 ginous). 



