Fishes of the Western North Atlantic 



97 



9b. Caudal fin occupies considerably less than y> total length. 



lOa. 5th gill opening well in front of origin of pectoral; eye 

 without nictitating fold or membrane. 

 1 1 a. Jaws widely protrusible forward; snout greatly 

 elongate. Scapanorhynchidae, p. 109. 



I lb. Jaws not widely protrusible ; snout not greatly elon- 

 gate. Carchariidae, p. 98. 



Figure 12. A Eye of Sfhyrna diflana, about 1375 

 mm. long, to show nictitating membrane (U. S. Nat. 

 Mus., No. 108452), about 2 x natural size. B Eye of 

 Mustelus canis, about three feet long, to show sub- 

 ocular fold (Harv. Mus. Comp. Zool., No. 35245). 



lOb. 5th gill opening over or behind origin of pectoral; eye 



with a more or less strongly developed nictitating fold or 



membrane. 



1 2a. Upper edge of nictitating fold continuous with edge 

 of eyelid, or even arising outside latter posteriorly, 

 although enclosing it anteriorly; teeth low, rounded 

 or with 3 or more cusps, usually in mosaic arrange- 

 ment, several series functional simultaneously in 

 sides of jaws as well as in front. 



Triakidae, p. 233.^° 



12b. Upper edge of nictitating membrane arises far 

 within edge of eyelid posteriorly, as well as ante- 

 riorly; teeth blade-like with i cusp only, not in 

 mosaic arrangement, usually not more than i or 2 

 series functional in sides of jaws simultaneously. 



Carcharhinidae, p. 262.'° 



10. It may not be possible to draw a sharp line between Triakidae and Carcharhinidae with respect to the nictitating 

 membrane or the teeth. However, the definition given above will serve to place any genus yet known from the 

 Atlantic in the one family or in the other. 



