584 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 115 



that on lower jaw, which is not visible when the mouth is closed; 

 pectoral fin origin below fourth to third gill-opening; tip of posterior 

 margin of first dorsal fin not reaching beyond level of pelvic origin; 

 distal tip of appressed pectoral fin not reaching much, if any, beyond 

 level of first dorsal origin; hyomandibular pores usually not forming 

 a discrete enlarged series on either side of the corner of the mouth 

 (pores in the series enlarged posteriorly, but gradually, rather than 

 abruptly, as in Rhizoprionodon) ; pores frequently not distinguishable. 

 Other important distinguishing characters as in key couplet 2a (p. 068). 

 (Note: ioaroc^oTi was described as having a small spiracle. In embryos 

 a tiny spiracle is present, but it is so small that to distinguish it from 

 adjacent pores of the lateral line system is hardly possible. In adults 

 the spiracle is not noticeable to an untrained observer. It is for this 

 reason that Loxodon specimens most often have been referred to 

 "Scoliodon" species (not S. laticaudus). For practical purposes, the 

 spiracle in Loxodon is absent. Some specimens of Scoliodon and 

 Rhizoprionodon have noticeably enlarged pores in the spiracular region. 

 One of these could possibly be the remnant of a spiracle, but I have 

 been unable to demonstrate this.) 



The shallow notch in the posterior rim of the orbit is always present 

 though it may not always be obvious to an untrained observer unless 

 compared with the smooth orbital rims of other species (see fig. 4). 

 Occasionally there are two notches or an area around the notch that 

 appears to be scarred. The flap on the nostril is frequently triangular 

 without a developed nipple-like lobe. The labial furrows are very 

 short and, for practical purposes, absent on the upper jaw; however, 

 in one specimen examined, the upper labial furrow on one side was 

 present and comparable to that of some specimens of Rhizoprionodon. 

 The distance from the tip of the upper caudal lobe to the subterminal 

 notch is generally much greater and slenderer than in the other species 

 treated in this paper, but sometimes it approaches the size and form 

 as found in Rhizoprionodon (figure 4a illustrates one such specimen). 

 Both precaudal pits are present but the lower is not as well developed 

 as the upper. The body and fins have a generally slenderer appearance 

 than in Rhizoprionodon. 



Growth changes: Snout length to origin of pectoral fin, pelvic fin, 

 first dorsal fin, second dorsal fin, anal fin, upper and lower caudal lobes 

 tend, in general, to increase in percent of t.l. with increased t.l. 



Eye diameter, length of upper caudal lobe, and distance from tip of 

 upper caudal lobe to notch tend to decrease in percent of t.l. with 

 increased t.l. 



From the few specimens examined it does not appear that there is 

 much, if any, change in the relation of the axis of the upper caudal 

 lobe with the horizontal axis of the body with increased t.l. 



