678 PROCEEDINGS OF THE NATIONAL MUSEUM vol. iis 



Pacific between 150° W. and 160° W., Aug. 26, 1953, USFWS, Pacific 

 Oceanic Fishery Investigations (MV John R. Manning, Cruise 16, 

 Station 29); also measurements of male, 2,150 mm., from Pacific, 

 north of Hawaiian Islands, between 22° N. and 47° N., and 159° W. 

 and 177° W., Sept. 19, 1954, USFWS, Pacific Oceanic Fishery In- 

 vestigations (MV John R. Manning, Cruise 22, Station 7). 



Diagnosis. — An Isurus with very long pectoral fins equal in length 

 to head; with at least some dusky coloration on underside of snout 

 and around mouth; and with first tooth on each side of symphysis in 

 both jaws having a complete cutting edge (from tip of cusp to base) 

 on its lateral as well as on its medial margin. 



Comparison. — The very long pectoral fins of /. alatus readily dis- 

 tinguish this species from /. oxyrinchus. As can be seen in figure 1, the 

 pectoral fins of I. alatus are equal to the head length, whereas in /. 

 oxyrinchus they are not more than about 70 percent of the head length 

 over most of the size range of specimens examined. Only in specimens 

 of /. oxyrinchus of 2,400 mm. long or longer do the pectorals exceed 70 

 percent of the head length, and the highest value I have is 84 percent 

 in a New Zealand specimen 2,642 mm. long. 



Comparison of pectoral length as a percentage of total length relative 

 to total length (fig.2) again indicates a clear-cut separation of /. alatus 

 and 7. oxyrinchus. However, the relative increase in pectoral length 

 with growth in specimens of /. oxyrinchus of 1,200 mm. long and longer 

 does mean that the largest adults of /. oxyrinchus approximate, but do 

 not overlap, /. alatus. Information is desirable on larger specimens of 

 /. alatus to indicate whether it shows a comparable relative increase in 

 its pectoral length with growth. The apparent relative decrease in 

 pectoral fin length which my data (fig. 2) show for specimens of I. 

 oxyrinchus from birth size to about 1,000 mm. is, if vahd, another in- 

 teresting example of the variation in growth-change patterns in sharks. 



As well as its longer pectoral fins, I. alatus has longer pelvic fins than 

 /. oxyrinchus (figs. 6, 7; table 2). In /. alatus the anterior margin of 

 the pelvic fin is equal to or only slightly shorter than the distal mar- 

 gin, whereas in /. oxyrinchus the anterior margin is noticeably shorter. 



The dusky coloration on the underside of the snout of I. alatus con- 

 trasts strongly with the immaculate whiteness of this region in /. 

 oxyrinchus. The duskiness increases with growth — hence the smallest 

 specimens of I. alatus only have duskiness fringing the underside of the 

 snout tip and aroimd the mouth. In the largest specimen the duski- 

 ness is not only more extensive under the snout but is darker. 



Several differences can be noted between the teeth of /. alatus and 

 /. oxyrinchus. Those of I. alatus have noticeably broader cusps, are 

 less flexuous, and less oblique (pis. 1, 2; figs. 8, 9). The first upper and 

 lower teeth of I. alatus have on their lateral margins a cutting edge 



