xni 



PHYLUM OHORDATA 201 



ossified by three bones : an epihyal (E. HY.) above, then a large 

 ceratohyal (C. HY.), and below a small double hypohijal (H. HY.). 

 The right and left hyoid bars are connected by a keystone-piece, 

 the unpaired, toothed basihyal (B. HY.), which supports the tongue. 



Connected with the hyomandibular and hyoid cornu are certain 

 investing bones serving for the support of the operculum. The 

 opercular (Fig. 870, op.) is articulated with a backward process of 

 the hyomandibular ; the pre-opercular (praop.) lies outside the 

 posterior border of the hyomandibular and quadrate, and clamps 

 them together ; the sub-opercular (subop.) is below and internal to 

 the opercular ; and the inter -opercular (intop.) fits between the 

 lower portions of the three preceding bones, and is attached by 

 ligament to the angle of the mandible. The twelve sabre-shaped 

 branchiostegal rays (branchiost.) are attached along the posterior 

 border of the epi- and cerato-hyal, and below the basi-hyal is an 

 unpaired bone, the basibranchiostegal or urohyal. 



There are five branchial arches, diminishing in size from before 

 backwards (Fig. 871). The first three present the same segments as in 

 the Dog-fish : pharyngobranchial (PH. BR.) above, then epibranchial 

 (E. BR.), then a large ceratobranchial (C. BR.), and a small hypo- 

 branchial (H. BR.) below. The right and left hypobranchials of 

 each arch are connected by an unpaired basibranchial (B. BR.). 

 All these segments are ossified by replacing bones, and the basi- 

 branchials are connected with one another and with the basihyal 

 by cartilage, so as to form a median ventral bar in the floor of the 

 pharynx. In the fourth arch the pharyngobranchial is unossified, 

 and the hypobranchial absent, and the fifth arch (BR. 5) is reduced 

 to a single bone on each side. Small spine-like ossifications are 

 attached in a single or double row along the inner aspect of each of 

 the first four arches : these are the gill-rakers ; they serve as a 

 sieve to prevent the escape of food by the gill-slits. 



The comparison of this singularly complex skull with the com- 

 paratively simple one of the Dog-fish is much facilitated by the 

 examination of the skull of a young Trout or Salmon. In the 

 latter, at about the second week after hatching, the only ossifications 

 present are a few investing bones ; when these are removed we get 

 a purely cartilaginous skull (Fig. 872), exactly comparable with 

 that of an Elasmobranch. There is a cranium devoid of replacing 

 bones and divisible only into regions ; the upper jaw is an unossified 

 palatoquadrate (PI. Pt., M. Pt., Qu.) and the lower jaw (Mck.) a 

 large Meckel's cartilage ; the suspensorium is an undivided 

 hyomandibular (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 proximal series consists of slender bony rays 

 the interspinous bones (Fig. 876, PTG. ; Fig. 873, PTG. I), lying 



