VERTEBRAL NUMBERS IN SHARKS — SPRINGER AND GARRICK 75 



Drs. Bruce B. Collette, Carl L. Hubbs, W. Ralph Taylor, and S. J. 

 VVeitzman read the manuscript and made valuable suggestions for its 

 improvement. 



Methods 



Sharks ranging in size from small embryos to specimens several feet 

 long were used in the survey. Radiographs were made with a hard- 

 ray machine and various industrial X-ray films. The finest-grained 

 films gave the most desirable results, but generally grain size was a 

 limiting factor only in making counts of the terminal vertebrae in the 

 caudal fin. A few counts were made on skeletons. Each vertebral 

 count was separated into two parts: 



1. Precaudal vertebral count (P) includes all complete centra 

 anterior to the forward edge of the upper precaudal pit, or, in species 

 where a pit is absent, all complete centra anterior to the origin of the 

 upper lobe of the caudal fin. 



2. Caudal vertebral count (C) includes all centra posterior to the 

 precaudal vertebrae. 



In order to demarcate clearly the precaudal from caudal centra on a 

 radiograph, a pin was inserted at the forward edge of the upper pre- 

 caudal pit, or at the upper caudal origin, so that its point touched the 

 vertebral column. In some sharks, notably species of Brachaelums, 

 Halaelurus, and Hemiscyllium, it is impossible to decide the point of 

 origin of the dorsal lobe of the caudal fin; for these sharks, only total 

 counts (T) are given. 



We do not know if our two methods of separating precaudal from 

 caudal vertebrae produce homologus (hence comparative) counts in 

 sharks with and without precaudal pits; however, the value of the 

 methods lies in their usefulness and for the purposes of this study we 

 consider the results homologous. Another possible way of subdivid- 

 ing the vertebral column would be into monospondylous and diplo- 

 spondylous centra.^ The transition from monospondyly (anterior) to 

 diplospondyly (posterior) usually occurs above the pelvic fin, but there 

 are notable exceptions (Etmopterus and a few species of CarcJiarhinus) 

 wherein it is much further posterior. The transition usually is evi- 

 denced by an abrupt reduction in centrum length (pi. 1a, b)^ but in 

 several species, e.g., Alopias superciliosus (pi. Ic), Scoliodon laticaudus, 

 Prionace glauca, the reduction is so slight as to be unnoticeable on a 

 radiograph. 



2 Monospondyly=one centrum per myomere; diplospondyly=two centra per myomere. For detailed 

 discussion, see Goodrich, 1930, pp. 26-27. 



• We have not made dissections to confirm the transition points from monospondyly to diplospondyly 

 but accept the first notably shorter centrum above or behind the pelvic region as the first diplospondylous 

 centrum. Confirmation of this view is found in some radiographs wherein it is possible to see the apertures 

 for nerve roots Issuing: one pair for each centrum In the monospondylous region and one pair for two centra 

 In the diplospondylous region. 



