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The difference between the species in the shape of the tail, especially 

 in the relative prominence of the terminal lobe of the dorsal lobe and 

 the sLibterminal notch (figs. 6, 7), may not be so evident in large 

 specimens. Usually the terminal lobe becomes smaller and less con- 

 spicuous with growth. If this is so in /. alatus, then adults of /. 

 alatus should show a condition more like that of /. oxyrinchus. 



The relative slenderness of /. alatus is difficult to evaluate because 

 my material is in poor condition. However, my interpretation of 7. 

 alatus as a slender species compared mth I. oxyrinchus is supported 

 by Fourmanoir's data (1961, pp. 17, 79) which give the weight of a 

 2,180 mm. specimen, apparently /. alatus, as 70 kilograms, whereas 

 two specimens of 7. oxyrinchus of 2,580 mm. weighed 150 and 200 

 kilograms. Bigelow and Schroeder (1948, p. 128) give the weight of 

 an 1,830 mm. C. oxyrinchus as 61 kilograms, and a 2,340 mm. specimen 

 as 104 kilograms. 



Figure 8.—Isurus alatus, new species, USNM 196039, male, 1801 mm., central Pacific: a, b, 

 upper and lower teeth, right side; c, d, first upper tooth, right side, anterior and lateral 

 views, showing complete cutting edge on lateral margin. 



One further difference between the two species is in the size of the 

 eye, that of 7. alatus being slightly larger at all sizes than that of 7. 

 oxyrinchus. 



Judging by the photographs of two central Pacific specimens (pis. 3,4), 

 the first dorsal fin of 7. alatus changes in shape and proportions \\dth 

 growth in a manner comparable to that of 7. oxyrinchus. 



Description.— Based mostly on the 1,233 mm. holotype and the 

 1,372 mm. para type. 



For proportional dimensions in percent of total length, see table 2. 



