REVISION OF CARCHARHINID SHARK GENERA — SPRINGER 567 



(sometimes a series consists of a single tooth) and extremely difficult 

 to count. Competent colleagues, not engaged in elasmobranch 

 studies, were given specimens on which to count the teeth and more 

 often than not made significant errors. Caution, as well as magni- 

 fication and good illumination, is recommended strongly for such 

 counts. 



Enlarged hyomandibular pores : In the genus Rhizoprionodon, there 

 is a discrete group of comparatively large and noticeable pores of 

 the hyomandibular series opening near the upper labial furrow and 

 extending dorsoposteriorly a short distance (see figures of the various 

 species). Frequently these pores are in a straight line, sometimes 

 irregularly biserial. Some difficulty in counting was found when 

 the pores were clogged or when they had not broken through the 

 denticles (in small specimens). Counts should be made under 

 magnification and any large gaps between pores should be searched 

 for obscured members of the series. Sometimes the most anterior 

 pore is well separated from the remaining ones. Scoliodon may have 

 a few enlarged pores, but they are frequently difficult to find. (See 

 also description of Loxodon.) 



Vertebrae : Radiographs of sharks were made using a very fine-grain 

 industrial film. The vertebral count was separated into two parts: 

 (1) precaudal vertebrae, which includes all complete centra anterior 

 to the forward edge of the upper precaudal pit; (2) caudal vertebrae, 

 which includes those centra posterior to the precaudal vertebrae. 



Precaudal vertebral counts are sometimes subject to an error of 

 plus or minus two vertebrae. One reason for this is that to establish 

 accurately the position of the first vertebra is not always possible 

 because of masking by other structm*es behind the head, including 

 the occipital condyles. Another is that sometimes an error in estab- 

 lishing the last precaudal vertebra occurs because of the angle with 

 which the X-rays entered the body in the region of the precaudal pit; 

 a two-dimensional parallax is created on the film. This last problem 

 was circumvented in most instances by sticking a pin, to mark the 

 first caudal vertebra, into the vertebral column immediately posterior 

 to the anterior margin of the precaudal pit. The errors affect the 

 counts very little. 



At the tip of the caudal fin the last few vertebrae are frequently 

 and irregularly fused (each fusion was counted as one vertebra), or 

 they are too small to give good resolution on the film. Caudal verte- 

 bral counts were made under high illumination and magnification, 

 and they are reasonably accurate for the purposes of this study. 



In embryos, precaudal vertebrae are formed early, but caudal 

 vertebrae are not completely formed until shortly before birth; 

 therefore, precaudal, but not caudal, vertebral counts from embryos 

 can be used (in Loxodon, for which I had few specimens, caudal counts 



