PISCES 



119 



67, A). Its terminal mouth and almost diphycercal caudal fin are 

 decidedly archaic. Whether its frilled branchial clefts are primitive 

 or specialized it is impossible to say. 



EXTINCT ELASMOBRANCHS 



Some of the extinct families of sharks: Cladoselachidse, Pleura- 

 canthidse, Diplacanthidae, and Acanthodidae, bring to light certain 

 conditions that are obviously more primitive than those of any of the 

 modern elasmobranchs. They all have the mouth terminal instead 

 of ventral, indicating that the terminal mouth which characterizes 

 most of the Teleostomi is more primitive than the ventral mouth 



FIG. 65. Cladoselache fyleri; Upper Devonian, Ohio. A, right side view; B, 

 ventral view; C, front view; restored. (From Lankester, after Woodward.) 



of the modern elasmobranchs. There are no claspers on the ven- 

 tral (pelvic) fins, indicating that the habit of copulation, with internal 

 incubation of eggs and consequent viviparity, so common among 

 modern elasmobranchs, is a caenogenetic specialization. The noto- 

 chord persists as an unbroken elastic rod and the neural and haemal 

 arches are developed only about as far as they are in the Cyclostomata. 

 The exoskeletal elements appear to be wanting in Pleuracanthidce and, 

 in the other families, are smaller and less developed than in modern 

 sharks. 



Each of these families shows some one or two features more prim- 

 itive than the others. One of the most striking features shown by 



