xin PHYLUM CHORDATA 207 



mediation of a small, rod-like bone, the interhyal (i. HY.), which 

 perhaps represents the hyomandibular of Elasmobranchs. It is 

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

 ceratohyal (c. HY.), and below a small double hypohyal (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 

 opcrcular (Fig. 859, op.) is articulated with a backward process of 

 the hyomandibular, the preopercular (pra. op.} lies outside the 

 posterior border of the hyomandibular and quadrate, and clamps 

 them together ; the subopercular (sub. op.) is below and internal to 

 the opercular; and the interopcrcular (int. op.) fits between the 

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

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

 IrancTiiostegal rays (branchiost.) are attached along the posterior 

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

 unpaired bone, the basi-branchiostegal or urohyal. 



There are five branchial arches, diminishing in size from before 

 backwards. The first three present the same segments as in the 

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

 (E. BR.), then a large ceratobrancliial (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 pharyngo-branchial 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 861), 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 palato- 

 quadrate (PL Pt., M. Pt., Qn.) and the lower jaw (Mel:.} a large 

 Meckel's cartilage ; the suspensorium is an undivided hyoman- 

 dibular (HM.\ and the hyoid and branchial arches are unseg- 

 mented. 



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



