112 Memoir Sears Foundation for Marine Research 



Key to Species of the Northern Hemisphere' 



I a. Distance from tip of snout to anterior edge of eye at least V2 as great as from posterior 

 edge of eye to ist gill opening, each measurement taken between perpendiculars; 

 lower surface plain-colored, without dark blotches. 



nasus (Bonnaterre), 1788, p. 112. 



lb. Distance from tip of snout to anterior edge of eye less than Vs as great as from pos- 

 terior edge of eye to ist gill opening; lower surface conspicuously marked with dark 

 blotches, except perhaps in very young specimens, diiropis Hubbs and Follett, 1 947. 



Warm Temperate to Subboreal Belt, Both 

 Sides of North Pacific. 



Lamna nasus (Bonnaterre), 1788 



Mackerel Shark, Porbeagle; Blue Shark (in Gulf of Maine) 



Figures 15, 16, 17 



Study Material. Male, 935 mm., from Nahant, Mass. (Harv. Mus. Comp. Zool., 

 No. 209) ; specimens of 660, 963 and 966 mm., from New England, North Atlantic and 

 Continental Slope off Sable I., Lat. 42° 37' N., Long. 60° 55' W. (U.S. Nat. Mus., No. 

 47528, 24288, 44057); head, from 70 miles SE. of Cape May (U.S. Nat. Mus., No. 

 125884) ; two embryos, no doubt of this species, about 180 mm. long, from Barnstable, 

 Mass., taken in October 1942 (Harv. Mus. Comp. Zool., No. 35901); also eight speci- 

 mens fresh caught in Gulf of Maine (not preserved) ; jaws from several of same; photo- 

 graph of i8o-mm. embryo from female caught off Portland, Maine, at "Mistaken 

 Ground" in January 1927 by Capt. D. C. Train. 



Distinctive Characters. Easily separable from the Sharp-nosed Mackerel Shark by 

 its teeth (cf. Fig. 16 D with 19) ; from Carcharodon by the teeth (cf. Fig. 16 D with 20 B) 

 and by the relative positions of the second dorsal and anal fins. 



Description. Proportional dimensions in per cent of total lengths. Male, 935 mm., 

 from Nahant, Mass. (Harv. Mus. Comp. Zool., No. 209). 



Trunk at origin of pectoral: hrea.dth 15.O; height 14.5. 



Snout length in front of: outer nostrils 6.0; mouth 7.1. 



Eye: horizontal diameter 2.4. 



Mouth: breadth 8.1 ; height S-5- 



Nostrils: distance between inner ends 3.6. 



Labial furrow length: upper 2.2; lower 1.3. 



Gill opening lengths : ist 7.5; 2nd 7.1 ; 3rd 6.5; 4th 6.$) 5th 6.5. 



First dorsal fin: vertical height i i.O; length of base 9.1. 



Second dorsal fin: vertical height 2.7 ; length of base 1.7. 



Anal fin: vertical height 2.7; length of base 1.6. 



8. It would be premature to expand the Key to the southern hemisphere for the reasons stated above. 



