XIII 



PHYLUM CHORDATA 191 



furnishing roofs to the orbits. Immediately behind the frontals is 

 a pair of very small parietals (PA.}, in front of them is an unpaired 

 supra-ethmoid (S. ETH.}, to the sides of which are attached a pair 

 of small nasals (NA.). On the ventral surface is the large para- 

 sphenoid (PA. SPH.), which forms a kind of clamp to the whole 

 cartilaginous skull floor; and in front of and below the parasphenoid 

 is the toothed vomer ( VO.}. Encircling the orbit is a ring of scale- 

 like bones, the sub-orbitals. (Fig. 811, o.}. 



In the jaws, as in the cranium, we may distinguish between 

 primary and secondary structures. The primary upper jaw or 

 palato-guadrate is homologous with the upper jaw of the Dog-fish, 

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

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

 the olfactory capsule : then the pterygoid (PTG.) on the ventral 

 and the meso-pterygoid (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 

 meta-pterygoid (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 membrane 

 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, the j-v.gal (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 cartilage 

 bone. Ensheathing Meckel's cartilage and forming the main part 

 of the secondary lower jaw is a large toothed membrane bone, the 

 dcntary (DNT}, and a small membrane 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 liyo- 

 mandibular (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 palato-quadrate 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 hyo-mandibular (to- 

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



