SHARK GENUS ISURUS — GARRICK 



679 



which extends from the tip of the cusp to the base. The medial mar- 

 gins are similarly armed. The first tooth is therefore similar to the 

 second tooth. In /. oxyrinchus (excepting largest adults) the first 

 tooth in each jaw does not have a complete cutting edge on its lateral 

 margin. It is therefore dissimilar from the second tooth, particularly 

 in the upper jaw. The cutting edge on the lateral margin of the first 

 tooth is present at the tip of the cusp but extends only part way to the 

 base. The relative length of this cutting edge varies with growth. In juve- 

 niles the cutting edge may be only one-third or less of the length of the 

 cusp, but in subadults and larger specimens it will commonly extend 

 along two-thkds of the cusp. In large adults of 2,500 mm. or more 

 the cutting edge approaches or reaches the base— when the distinction 

 between /. alatus and /. oxyrinchus on this character disappears. The 

 same can be said of tooth shape in large adults of /. oxyrinchus where 

 the teeth are less flexuous and proportionately broader than those of 

 smaUer specunens— thus the teeth of large adults of /. oxyrinchus are 

 like those of /. alatus. 



Figure 7-Isutus alatus, new spedes: a, b, holotype, USNM 197427, male, 1233 mm., 

 tropical Pacific or Indian Oceans. 



Another difference between the teeth of the two species is in the 

 position and shape of the third upper tooth. In /. alatus the thu-d 

 upper tooth is below the lateral margin of the palatine process of the 

 palatopterygoquadrate cartilage and is only shghtly obhque in shape 

 (pi. 1). In 7. oxyrinchus the third upper tooth is more medial in 

 position, usually below the upper two-thirds of the lateral margin of 

 the palatine process, and the tooth itself is more asymmetrical and 

 oblique in shape. 



