560 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 115 



single genus and separating them primarily from Carcharhinus have 

 never been adequately evaluated. These characters are the smooth 

 and oblique teeth, the nature of the labial furrows, and, in Loxodon, 

 the presence of a spiracle (see diagnosis under Loxodon). In addition 

 to these characters, and more important, I find that the group can 

 be conveniently, though not naturally, separated from other carcha- 

 rhinids in having the origin of the second dorsal fin never in advance 

 of the middle of the anal base and frequently over or behind the anal 

 axil. The group differs from most carcharhinids in having the cusps 

 of most teeth smooth although in the larger specimens of some Rhizo- 

 prionodon species the cusps become serrulate. 



These characters are not unique; a few other carcharhinids, e.g., 

 Carcharhinus porosus (Ranzani) and C. borneensis (Bleeker), have a 

 posteriorly placed second dorsal origin (but never posterior to a ver- 

 tical line through the middle of the anal base). These species, how- 

 ever, have noticeably serrated teeth, the upper teeth possessing very 

 large basal serrations on the posterior (lateral) margins, and thus differ 

 from all the species treated in the present study. Because Carcha- 

 rhinus poros^is and C. borneensis lack a long upper labial furrow, they 

 might be confused superficially with Scoliodon laticaudus, Loxodon 

 macrorhinus, Rhizoprionodon oligolinx, or R. taylori. Aside from the 

 nature of the teeth, Scoliodon (q.v.) can be distinguished by its unique 

 fin positions; Loxodon (q.v.), by its short dorsal fin base, by the second 

 dorsal fin usually originating posterior to a vertical line through the 

 anal axil, and by having a notched orbital rim; Rhizoprionodon oligo- 

 linx and R. taylori, by having the origin of the second dorsal fin over, 

 or posterior to, a vertical line through the posterior third of the 

 anal base. 



Several species assigned to the genus Scoliodon have been described 

 from fossil teeth. I choose not to treat them here as I believe the 

 affinities of fossils based only on teeth are at present indecipherable 

 and possibly will remain so in most cases. This viewpoint is based on 

 the close similarity of teeth within the living genera included in the 

 present study, as well as a similarity to the teeth of the distantly 

 related Sphyrnidae. The problem is further confused by the 

 heretofore unrecognized dental sexual dimorphism that is found in 

 several carcharhinids, including Rhizoprionodon and Scoliodon. Re- 

 lationships presently based only on teeth, therefore, are open to 

 question. 



The species of Scoliodon, Loxodon, and Rhizoprionodon are dis- 

 tributed primarily through the shallow tropical marine waters of the 

 world, with some forms straying over moderate depths and into tem- 

 perate areas. None of the species is known to occur in the Mediter- 

 ranean or in Oceania. 



