192 ZOOLOGY SECT. 



end of the hyoid arch and the homologue of the hyo-mandibular 

 of Elasmobranchs, but there is some reason for thinking that it 

 really belongs to the mandibular arch, and corresponds with the 

 dorsal and posterior part of the triangular palato-quadrate of 

 Holocephali : a perforation in the latter would convert it into an 

 inverted arch having the same general relations as the upper jaw 

 plus suspensorium of the Trout, but fused, instead of articulated, 

 with the cranium at either extremity. 



The hyoid cornu is articulated to the cartilaginous interval 

 between the hyo-mandibular and symplectic through the inter- 

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

 perhaps represents the hyo-mandibular of Elasmobranchs. It is 

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

 cerato-Jiyal (c. HY.), and below a small double hypo-hyal (H. HY.). 

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

 the unpaired, toothed lasi-hyaL (B. HY.), which supports the 

 tongue. 



Connected with the hyo=mandibular and hyoid cornu are certain 

 membrane-bones serving for the support of the operculum. The 

 apercular (Fig. 811, op.} is articulated with a backward process of 

 the hyo-mandibular, the pre-opercular (pra-op.) lies outside the 

 posterior border of the hyo-mandibular and quadrate, and clamps 

 them together ; the sub-opercular (sub-op} is below and internal to 

 the opercular; and the inter -opercular (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 

 branchiostegal rays (branckiost.) are attached along the posterior 

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

 unpaired bone, the basi-brancliiostegal or uro-liyal. 

 * There are five branchial arches, diminishing in size from before 

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

 Dog-fish : pharyngo-'branchial (PH. BR.) above, then cpi-branchial 

 (E. BR.), then a large cerato-branchial (c. BR.), and a small hypo- 

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

 each arch are connected by an unpaired 'basi-lranchial (B. BR.). 

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

 branchials are connected with one another and with the basi-hyal 

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

 pharynx. In the fourth arch the pharyngo-branchial is unossi- 

 fied, and the hypo-branchial absent, and the fifth arch (BR.5) is 

 reduced to a single bone on each side. Small spine-like ossifica- 

 tions 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 



