206 ZOOLOGY SECT. 



palatoquadrate is homologous with the upper jaw of the Dog-fish, 

 but instead of remaining cartilaginous, it is ossified by five replac- 

 ing bones : the toothed palatine (PAL.) in front, articulating with 

 the olfactory capsule: then the pterijgoid (PTG.) on the ventral, 

 and the mesopterygoid (MS. PTG.) on the dorsal edge of the 

 original cartilaginous bar: the quadrate (QU.) at the posterior 

 end of the latter, furnishing a convex condyle for the articulation 

 of the lower jaw : and projecting upwards from the quadrate the 

 metapterygoid (MT. PTG.). These bones do not, however, enter 

 into the gape, and do not therefore constitute the actual upper 

 jaw of the adult fish : external to them are two large investing 

 bones, the premaxilla (PMX.) and the maxilla (MX.), which 

 together form the actual or secondary upper jaw ; they both bear 

 teeth. A small scale-like bone, ihejugal (JU.) is attached to the 

 posterior end of the maxilla. 



The lower jaw is similarly modified. Articulating with the 

 quadrate is a large bone, the articular (ART.), continued forwards 

 by a narrow pointed rod of cartilage : the latter is the unossified 

 distal end of the primary lower jaw or Meckel's cartilage ; the 

 articular is its ossified proximal end, and therefore a replacing bone. 

 Ensheathing Meckel's cartilage and forming the main part of the 

 secondary lower jaw is a large toothed investing bone, the dentary 

 (DNT.), and a small investing bone, the angular (ANG.) is 

 attached to the lower and hinder end of the articular. 



The connection of the upper jaw with the cranium is effected 

 partly by the articulation of the palatine with the olfactory region, 

 partly by means of a suspensorium formed of two bones separated 

 by a cartilaginous interval : the larger, usually called the hyoman- 

 dibular (HY. M.), articulates with the auditory capsule by the 

 facet already noticed, and the small pointed symplectic (SYM.) fits 

 into a groove in the quadrate. Both bones are attached by 

 fibrous tissue to the quadrate and metapterygoid, and in this way 

 the suspensorium and palatoquadrate together form an inverted 

 arch, freely articulated in front with the olfactory, and behind 

 with the auditory capsule, and thus giving rise to an extremely 

 mobile upper jaw. As its name implies, the hyomandibular (to- 

 gether with the symplectic) is commonly held to be the upper 

 end of the hyoid arch and the homologue of the hyomandibular 

 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 palatoquadrate 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 hyomandibular and symplectic through the inter- 



