114 TBE ANATOMY OF VERTEBRATED ANIMALS. 



ter consists of a lateral arch upon each side, united with its 

 fellow, and with the branchial arches, by the intermediation 

 of medial basal elements below ; and it is succeeded by a 

 variable number of similar arches, which support the branchial 

 apparatus. 



From the hyoidean and from the branchial arches carti- 

 laginous filaments pass directly outward, and support the walls 

 of the branchial sacs. Superficial cartilages, which lie par- 

 allel with the. branchial arches, are sometimes superimposed 

 upon these. There are no opercular bones, though cartilagin- 

 ous filaments which take their place (Fig. 34, Op) may be 

 connected with the hyomandibular cartilage ; and, in the 

 great majority of the jElasmohranchii^ the apertures of the 

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

 Chimmra^ a great fold of membrane extends back from the 

 suspensorial apparatus, and hides the external gill-apertures. 



Large accessory cartilages, called labial^ are developed at 

 the sides of the gape in many Mlasmohranchii. (Figs. 34 and 



The pectoral arch consists of a single cartilage on each 

 side. The two become closel}^ united together in the ventral 

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

 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 'Vertebrata. 

 The pectoral fins, the structure of which has already been de- 

 scribed, are always the larger, and sometimes attain an enor- 

 mous 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 the}'' 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 more acute, the posterior more obtuse crowns. In the 

 Port Jackson shark {(Jestracion)^ however, the anterior teeth 

 are not more acute than the most obtuse teeth of the others, 

 while the middle teeth acquire broad, nearly flat, ridged 

 crowns, and the hindermost teeth are similar but smaller. 

 The Rays usually have somewhat obtusely-pointed teeth, but 

 in Myliohates^ the middle teeth have transversely-elongated, 



