SKATES 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. — Pristiophorus (cirratus). 2. (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 Rhznobatus (Fig. 100). The shark-like body 
form is here most nearly retained, and its fin structures 
Fig. 100.— Rhinobatus planiceps. 2. XX}. (After GARMAN.) (The lower 
portion of the figure showing ventral side.) S. 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 éarly as the 
Odlite. 
