XXIX TEETH 379 



calcified cartilage, situated just behind the last branchial 

 arch (Fig. 99, a, Sh. G). On each side of its outer surface 

 it presents three elevations or articular facets (Fig. 102, art./) 

 for the pectoral fin ; the presence of these allows of the divi- 

 sion of each side of the arch into a narrow, pointed dorsal 

 portion {pct.g), and a broader ventral portion {pct.g) united 

 in the middle line with its fellow of the opposite side. The 

 pectoral fin is formed of pterygiophores {rad), fused proxim- 

 ally to form basals {Bs. i. — 3), which are three in number, 

 and very large and strong. 



The pelvic girdle is a transverse bar of cartilage situated 

 just in front of the vent (Fig. 99, a, Felv. G), and present- 

 ing on its posterior edge facets for the pelvic fin. The latter 

 has the same general structure as the other fins, but has a 

 single very large basal cartilage, and its first or anterior 

 radial is also much enlarged. The free edges of both 

 pectoral and pelvic fins are supported by horn-like dermal 

 rays (Fig. 102, d.f.r). 



It will be noticed that while the skeleton of the crayfish 

 is a series of articulated tubes with the muscles inside 

 them, that of the dogfish is a series of articulated rods with 

 the muscles outside. The joints, formed by two rods 

 applied at their ends and bound together by ligament, are 

 not confined to movement in one plane, like the hinge- 

 joints of the crayfish, but are capable of more or less 

 rotatory movement. 



The mouth, as we have seen, is a transverse aperture 

 bounded by the upper and lower jaws. In the mucous 

 membrane covering the jaws are imbedded large numbers 

 of teeth, (Fig. 99, T) bony conical bodies, with enamelled 

 tips, arranged in transverse rows. They are to be looked 

 upon as special developments of the placoid scales or dermal 

 teeth, enlarged for the purpose of seizing prey. 



