164 



SELACH11 



The tail becomes very distinctly marked off, and slender ; the single 

 dorsal fin which alone remains not moving down towards the tip of 

 the tail, but being situated near its base. The pelvic girdle has 

 a median and no lateral prepubic process. 



Family TRYGONIDAE. The large rhombic disk is completed in front 

 of the skull by the pectoral fins, which meet in the "nddle line. 

 Their anterior axis rests on the preorbital process. The skeletal rostrum 

 is represented only by a median strand of connective tissue (Fig. 119). 

 A slender horseshoe-shaped basihyal is present ; but the basibranchials 

 are fused into a single large plate, apparently together with the hypo- 



FIG. 127. 



Dorsal view of Iteroplatm Valeneicnnii, Duni. (after Carman), showing the extensive 

 development of the lateral-line organs, dc, endolymphatic openings ; pv, pectoral fin plv 

 pelvic fin ; sc, spine ; up, spiracle. 



branchials. The ceratohyal is attached either to the skull directly or 

 to the base of the hyomandibular. The stout transverse jaws bear 

 numerous small and usually sharp teeth. The tail is slender, varying 

 much in development. In Urolophus it bears a terminal fin, and 

 in Tryyonoptera a dorsal fin ; but as a rule it is whip-like, and has 

 lost almost all trace of the fins. On the other hand, it is generally 

 provided with one or more large serrated spines (Fig. 127), whence their 

 name Sting-Rays. These spines are placed behind the fins, not in front 

 as in other Selachians. 



Xyphotrygon, Cope ; Eocene, N. America. Urolophus, M. and H., and 

 Trygon, Ad. ; warm seas, and to Eocene, Italy. Urogymmis, M. and H. ; 

 Indian seas. Pteroplatea, M. and H. (Figs. 92, 127); temperate and 

 tropical seas. Tr$gonoptera, M. and H. ; Australian. 



