568 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 115 



of embryos were used). In those instances wherein several embryonic 

 siblings or an embryo and its mother were available, none of the 

 embryonic counts are reported in the tables. If only the embryo 

 was available, the body count was included. 



In the genus Loxodon and the type subgenus of Rhizoprionodon, 

 the precaudal centra become gradually and markedly elongate 

 (plates 1 and 2) to a point above the region between the anus and 

 posterior pelvic base, where they become abruptly reduced in length. 

 The point at which reduction occurs is presumably the point where 

 diplospondyly begins. In Scoliodon and the subgenus Protozygaena 

 (plates 1 and 2), the precaudal centra remain more or less constant 

 in length and the point at which reduction takes place is noticeable 

 only on close scrutiny. Sometimes the diplospondyly is irregular 

 with elongate and reduced centra interspersed over a short distance. 



Key to Scoliodon, Loxodon, and Rhizoprionodon 



(Characters separating these three genera from other carcharhinid genera are 

 discussed in the introduction; characters delimited in the key are not repeated 

 in the generic or specific diagnoses and descriptions unless necessary.) 



la. Posterior tip of first dorsal fin extending to, or beyond (usually) , a vertical 

 line through the midbase of pelvic fin; origin of pectoral fin below, or only 

 slightly in advance of, fifth-gill opening; distal tip of appressed pectoral fin 

 over, or anterior (usually) to, its inner corner; snout tip to first dorsal 

 origin 34.7-40.8 percent of total length; length of gill-openings equal to, 

 or greater (usually) than, horizontal eye diameter; length of anal base 

 6.9-8.4 percent of total length; number of precaudal vertebrae exceeding 

 number of caudal vertebrae by 43-57 (one specimen out of 98 had only 35 

 more precaudal vertebrae than caudal vertebrae) (Indo-Pacific) . 



Scoliodon laticaudus 



lb. Posterior tip of first dorsal fin never extending much, if any. beyond a vertical 

 line through the origin of pelvic fin; origin of pectoral fin below fourth to 

 third gill-opening; distal tip of appressed pectoral fin posterior to its inner 

 corner; snout tip to first dorsal origin 27.0-34.0 percent of total length; 

 length of one or more gill-openings less than horizontal eye diameter; 

 length of anal base 3.3-5.4 percent of total length; number of precaudal 

 vertebrae ranging from 18 less than to 26 more than number of caudal 

 vertebrae 2 



2a. Posterior rim of orbit with a slight notch at midlevel; origin of dorsal fin 

 posterior to appressed pectoral inner corner by a distance* greater than 

 length of the fourth gill opening; eye larger at all sizes (compare table 6 

 with tables 4, 10, 11, 13-17) ; mouth small, its width 4.9-5.9 percent of total 

 length; its length 3.1-4.2 percent of total length; base of first dorsal fin 

 6.2-7.6 percent of total length; lengths of upper and lower labial furrows 

 combined is less than 1.5 percent of total length (Indo-Pacific). 



Loxodon macrorhinus 



2b. Posterior rim of orbit without a notch; origin of dorsal fin rarely posterior 

 to appressed pectoral inner corner by a distance as great as length of 

 fourth gill-opening, usually over, or in advance of, appressed pectoral inner 

 corner; eye smaller at all sizes; mouth larger, its width 6.2-7.8 percent of 



