THE ELASMOBRANCHS 6/ 



sort had to be available to the male for the intromission 

 of sperm into the cloaca of the female: in recent elasmo- 

 branchs these pelvic fins take the form of speciahzed 

 claspers carrying erectile tissue and so designed that they 

 can be inserted into the cloaca during copulation. Pelvic 

 fins and claspers are generally preserved in the fossil 

 record and it should be possible from a study of this 

 record to discover when the uremic habitus, with its 

 dependent mode of internal fertilization, was evolved. 

 The paleontologist, however, has afforded us httle infor- 

 mation on the point. True claspers are recorded in all 

 the Jurassic sharks, but among older forms they are de- 

 scribed only in the Cochliodontidae, which were ances- 

 tral to the chimaeroids, and in the fresh- water Pleura- 

 canthodii. Since both of these groups were derived from 

 a common stem (Cladodus), and in view of the fact 

 that the fine line between pelvic fins' and 'claspers' has 

 not been considered by the paleontologist, and since 

 what are recognized as 'pelvic fins' in the Carboniferous 

 forms may actually have been used as intromittent or- 

 gans, we infer that internal fertilization may go back to 

 the late Devonian, as suggested in Figure 6. In this view 

 the urea-retention habitus was probably the signal adap- 

 tation that enabled the Stegoselachii (which are be- 

 lieved to be ancestral to the Devonian elasmobranchs ) 

 to effect the first vertebrate invasion of the sea. By this 

 interpretation, the Carboniferous Pleuracanthodii, which 

 were inhabitants of fresh water and which possessed 

 claspers, must have returned to that habitat after the 

 urea-retention habitus had been acquired. 



There is no reason to believe that the elasmobranchs 

 have ever been irrevocably bound to salt water: the list 

 of those known to inhabit fresh water today, either tem- 

 porarily or permanently, comprises at least 13 famihes, 

 19 genera, and 22 species, and includes representative 

 sharks, rays, and skates. Experimental animals trans- 

 ferred to moderately diluted sea water show a marked 

 increase in urine flow, as is to be expected from the in- 



