SKA TES 



93 



sharks ; they appear comparatively late in time, and may 

 well be regarded as the culminating forms of the specializ- 

 ing bottom-living sharks of the Mesozoic. Whether they 

 are directly descended from forms like Squatina or Pristio- 



Fig. 99. — Pristiophoriis (cinatus). 5. (After JAEKEL.) Australia. 



phorus must be looked upon as exceedingly doubtful, as 

 the depressed body form may possibly have arisen 

 independently in these different families. The most 

 nearly ancestral form of the skates appears to have sur- 

 survived in RJnnobatits (Fig. lOO). The shark-like body 

 form is here most nearly retained, and its fin structures 



Fig. 100. — Rhinobatus planiceps. ?. x |. (After Garman.) (The lower 

 portion of the figure showing ventral side.) 5. Spiracle. GO. Gill slits. 



are the least specialized ; these transitional characters of 

 Rhinobatus become more prominent in view of its ancient 

 occurrence : its genus was clearly defined as early as the 

 Oolite. 



