THE ELASMOBRANCH FISHES 215 



or ceratohyal. The ceratohyal curves inward and forward, 

 running close inside the lower jaw, and is articulated to a 

 median crescentic plate of cartilage, known as the basihyal. 

 A number of slender cartilaginous rods called gill-rays, some 

 of which are branched, are borne on the posterior edges of 

 the hyomandibular and ceratohyal. The hyomandibular 

 cartilage not only carries the ventral segments of the by old 

 arch, but also supports the jaws, being bound to the posterior 

 edge of the upper jaw by a stout symplectic ligament. In 

 rays there is a pre-spiracular ligament passing from the 

 front of the auditory capsule, to the lower end of the 

 hyomandibular cartilage. This ligament is strengthened 

 by a curved piece of cartilage, stiffening the anterior wall of 

 the spiracle. The pre-spiracular ligament is absent in Scyllium 

 canicula ; a dense band of fibrous tissue which passes down 

 to the integument in front of the spiracle must not be 

 mistaken for it. The band in question is perforated by 

 a canal, and is connected with the mucous canal system. 

 It has no connection with the jaws. The five remaining 

 visceral arches form a series of cartilaginous hoops supporting 

 the roof, walls, and floor of the pharynx. They are known 

 as the branchial arches, and it should be noted that the 

 spiracle opens in front of the hyomandibular arch, the first 

 gill-slit (or branchial cleft) opens between the hyoid and 

 the first branchial arch, the second gill -slit between the 

 first and second branchial arches, and so on, so that each 

 branchial arch lies behind the gill -slit denoted by the 

 same number. The branchial arches are divided into 

 four segments on each side — namely, the pharyngo- 

 brauchials in the roof of the pharynx, the epibranchials and 

 ceratobranchials curving round and forming the supports of 

 the side walls of the pharynx, and the hypobranchials in the 

 floor of the pharynx. The first hypobranchial is a small 

 nodule of cartilage connecting the first ceratobranchial with 

 the basihyal. The second, third, and fourth hypobranchials 

 are directed backwards, and the two last are connected with a 

 median, dagger-shaped plate of cartilage, the basibranchial, 

 lying in the roof of the pericardial cavity. The fifth arch has 

 no hypobranchials, but its ceratobranchials are large and 

 expanded, joined to the basibranchial, and their posterior 

 borders are notched to admit the passage of the Cuvierian 



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