THE ELASMOBEANCHII. 129 



From the hyoidean and from tlie branchial arches carti- 

 laginous filaments pass directly outwards, and support the 

 walls of the branchial sacs. Superficial cartilages, which 

 lie parallel with the branchial arches, are sometimes super- 

 imposed upon these. There are no opercular bones, though 

 . cartilagiuous filaments which take their place (Fig. 34<, Op) 

 may be connected with the hyomandibular cartilage; and. 

 in the great majority of the Elasmobranchii, the apertures 

 of the gUl-sacs are completely exposed. But in one group, 

 the Chimcera, a great fold of membrane extends back from 

 the suspensorial apparatus, and hides the extei-nal gill- 

 apertures. 



Large accessory cartilages, called labial, are developed at 

 the sides of the gape in many Elasmohranchii. (Figs. 34 A. 

 35, i, k, I.) 



The pectoral arch consists of a single cartilage on each 

 side. The two become closely united together in the ventral 

 median line, and are not directly connected with the skvdl. 

 The pelvis is also represented by a pair of cartilages, which 

 may coalesce, and are invariably abdominal in position. 



There are always two pairs of lateral fins corresponding 

 with the anterior and posterior limbs of the higher Verte- 

 brata. The pectoral fins, the structure of which has alreadj' 

 been described, are always the larger, and sometimes attain 

 an enormous size relatively to the body. 



In these fishes, teeth are developed only upon the mucous 

 membrane which covers the palato-quadrate cartilage and 

 the mandible. They are never implanted in sockets, and 

 they vary greatly in form and in number. 



In the Sharks they are always numerous, and their crowns 

 are usually triangular and sharp, with or without serrations 

 and lateral cusps. As a rule, the anterior teeth on each side 

 have niore acute, the posterior more obtuse crowns. In the 

 Port Jackson shark {Cestracimi), however, the anterior teeth 

 are uot more acute than the most obtuse teeth of the others, 

 while the middle teeth acquire broad, nearly flat, ridged 

 crowns, and the hindei'most teeth are similar but smaller. 



K 



