1910.] AXATOMY OF THE FRILLED SHARK. 547 



Adductor mandibulce is a thick massive muscle, filling up the 

 concavities on the outer sides of the palatoquadrate and the 

 mandible. It is divided by an aponeurosis, which is in the line 

 of the mouth angle, and is attached above to the lower edge 

 of the palatoquadrate ridge previously mentioned. Below, it is 

 similarly attached to a correspondingly sharp ridge (rn.r.) on 

 the ventro-lateral border of the mandible. 



Constrictor superjicialis is a thin, extensive muscle lying behind 

 the hyomandibular. It extends round the jaw angle to the 

 ventral surface of the head, where it lies between the mandibles. 

 It is attached above to the posterior end of the supra-capsular 

 ridge (s.c.r.) and also to the outer surface of the hyomandibular. 

 On the lower side it is attached to the inner edge of the sharp 

 ridge (m.r.) which extends along the ventro-lateral border of the 

 mandible. 



d. Branchial Arches (Plate XLIII. fig. 6). 



Basibranchials. — The basibranchial of the first branchial arch 

 is, as previously mentioned, probably represented by the median 

 prominence (bbr. 1) on the posterior border of the basihyal. The 

 second one (bbr. 2) is situated considerably posterior to the first, 

 level, in fact, with the bases of the third ceratobranchials. It is 

 triangular in outline, attached anteriorly to the second pair of 

 hypobranchials (hbr. 2) and is free behind. The third (bbr. 3) is 

 also triangular in outline and is distinct. It is attached by 

 ligament anteriorly to the third pair of hypobranchials and is 

 firmly united behind by ligament to the fourth pair of hypo- 

 branchials. 



The fourth basibranchial is indistinguishable from the corres- 

 ponding hypobranchials. Garman describes it as lying between 

 these. However, in the two cases that I have examined it is 

 certainly not present, and I am inclined to believe that it has 

 become fused with the large posterior basibranchial. The latter 

 no doubt represents the fourth, fifth, and sixth basibranchials 

 together with their corresponding hypobranchials all fused into 

 one piece. It ends posteriorly in an elongate, tapering median 

 process. 



Hypobranchials. — The pair corresponding to the first arch are 

 represented by the lateral prominences (hbr. 1) on the posterior 

 border of the basihyal. The second pair (hbr. 2) are flat and 

 broad anteriorly, where they overlie the expanded part of the 

 lower ends of the first ceratobranchials. They are somewhat 

 thin there, and on the posterior edge are united by ligament to 

 their corresponding ceratobranchials. Towards their posterior 

 extremities they gradually taper and they are united to the 

 second basibranchial. The third pair (hbr. 3) is essentially 

 similar in shape to the second pair, the articulations being with 

 the corresponding ceratobranchials and basibranchial. The foux-th 

 pair (hbr. 4) is more rounded than the second and third, and each 

 hypobranchial has a bend in it so that the lateral part stands otit 



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