Author: Whitley, 1929
Requiem sharks with long, parabolic or subangular snouts, small, wide-spaced nostrils, no spiracles, labial furrows varying from short and confined to mouth corners to rather long but falling behind eyes, fairly large eyes, oblique- and narrow-cusped small teeth in both jaws, these with distal blades and serrations variably present or absent but without cusplets, no keels on caudal peduncle, transverse crescentic precaudal pits, second dorsal fin 1/3 height of first or less, second dorsal origin far behind anal origin and about opposite its insertion, anal fin with elongated preanal ridges and a straight or slightly concave posterior margin.
Body fairly slender to moderately stout. Head fairly broad, only moderately depressed, not trowel-shaped; snout narrowly to broadly parabolic or obtusely wedge-shaped in dorsoventral view, long, with preoral length greater than internarial space and mouth width; eyes rather large, without notches; spiracles absent; no papillose gillrakers on internal gill openings; nostrils small, internarial space about 3 to 5 times the nostril width; anterior nasal flaps very short, narrowly triangular, and not tubular; labial furrows short to rather long, with uppers shorter or longer than lowers falling far behind eyes; teeth similar in upper and lower jaws, anteroposteriors with slender oblique cusps and distal blades but no cusplets; serrations variably developed; cusps of lower teeth not prominently protruding when mouth is closed; 25 to 28/24 to 28 rows of teeth. Interdorsal ridge absent or rudimentary; no dermal keels present on caudal peduncle; upper precaudal pit transverse and crescentic. First dorsal origin usually over pectoral inner margins but sometimes slightly behind their rear tips, its midbase about equidistant between pectoral and pelvic bases or closer to the pectorals, and free rear tip usually anterior to pelvic origins but occasionally over them; second dorsal fin much smaller than first, its height 1/3 first dorsal height or less, its origin about over anal insertion; pectoral fins moderately broad and triangular, slightly falcate, pectoral length from origin to free rear tip 2/5 to 4/5 of pectoral anterior margin; pectoral origins varying from below third to below fourth gill slits; anal considerably larger than second dorsal, with very long preanal ridges and a straight or slightly concave posterior margin. Colour light grey, yellowish or brownishgrey above, without a colour pattern. Small sharks, adults not exceeding 1.3 to 1.5 m and most smaller than 1 m.
The arrangement of this genus follows V. Springer (1964) in most details, except that tile subgenus Protozygaena is not recognized (see Compagno, 1979 for a discussion of the classification and nomenclature of Rhizoprionodon and Protozygaena). Most of the species in Rhizoprionodon were formerly placed in Scoliodon, but the latter name is restricted to the very distinct S. laticaudus. The list of species recognized below is provisional. Particularly, the allopatric species pairs R. oligolinx- taylori and R. terraenovae-porosus are not sharply distinguished and may represent in each case clinal variation within a single wide-ranging species (Compagno, 1979). F. Sotelo (pers. comm.) is working on this problem in R. Terraenovae-porosus.