SKULL 3IQ 



The mandibular arch has two parts ; above the mouth is the 

 palatopterygoquadrate bar, which is hung from the auditory 

 capsule by a postspiracular ligament and from the cranium 

 by an ethmopalatine ligament, and which is joined to its fellow 

 in front by another ligament ; below the mouth is Meckel's 

 cartilage, articulating with the palatopterygoquadrate behind 

 and joined to its fellow by ligament in front. (An articulation is 

 a point where one cartilage or bone rests against another in such 

 a way that they can be moved relatively to each other.) At the 

 junction of the two cartilages ligaments join them to the hyo- 

 mandibula (see below) ; this is their chief connection with the 

 dorsal part of the skull. The arrangement by which the upper jaw 

 is slung from the skull by the hyomandibular is called a hyostylic 

 jaw suspension. This and other types of suspension are discussed 

 on pp. 582-83. Each of the remaining half-hoops consists of 

 a number of pieces of cartilage ; the general plan is for there 

 to be five of these, which are called, from above downwards, 

 pharyngo-, epi-, cerato-, hypo- and basi-branchial cartilages, the 

 last being a median piece, common to the two sides. Behind the 

 spiracle, which is considered to be a modified gill slit, lies the 

 hyoid arch, which has only three parts. The uppermost, called 

 the hyomandibula, or hyomandibular cartilage, is really the 

 epihyal ; it is, as we have already seen, attached to the jaws, 

 and above, it is firmly held by ligaments to the auditory capsule. 

 Below it, the ceratohyal passes forwards and joins the basihyal, 

 which lies in the floor of the mouth. Scylluim and Acanthias have 

 five pairs of gill slits, so that there are five more hoops, each called 

 a branchial arch, and the whole sometimes called the branchial 

 basket. (Other sharks have more gill sHts and so more arches, one 

 behind each.) The first hypobranchial joins the basihyal ; the 

 second hypobranchial meets its fellow and has no basi-element ; 

 the third and fourth hypobranchials join a single basibranchial; 

 the fifth hypobranchial is missing, but the fifth ceratobranchial 

 joins the basibranchial. The epi- and ceratobranchials are 

 distinguished from the other elements by bearing along their 

 hinder borders gill rays which support the gills. The two paired 

 elements of the hyoid also bear rays, showing their homology 

 with the epi- and cerato-elements. 



Outside the upper and lower jaws lie a pair of labial cartilages, 

 and along the outer sides of the second, third, and fourth cerato- 

 branchials are extrabranchials. 



