The Evolution of Fishes 447 



bear out the same idea. In all these the tail is less primitive 

 than in the Ichthyotomi. On the other hand, the vent in Cladose- 

 lache is thought by Dean to have been near the end of the tail. 

 If this is the case, it should indicate a very primitive character. 

 On the whole, though there is much to be said in favor of the 

 primitive nature of the Ichthyotomi (Pleuracanthus) with the 

 tapering tail and jointed pectoral fin of a dipnoan, and other 

 traits of a shark, yet, on the whole, Cladoselache is probably 

 nearer the origin of the shark-like forms. 



The relatively primitive sharks called Notidani have the 

 weakly ossified vertebrae joined together in pairs and there are 

 six or seven gill-openings. This group has persisted to our 

 day, the frilled shark (Chlamydoselachus) and the genera Hexan- 

 chus and Heptranchias still showing its archaic characters. 



Here the sharks diverge into two groups, the one with the 

 vertebrae better developed and its calcareous matter arranged 

 star-fashion. This forms Hasse's group of Asterospondyli, the 

 typical sharks. The earliest forms (Orodontida , Heterodontida) 

 approach the Notidani, and so far as geological records go, 

 precede all the other modern sharks. One such ancient type, 

 Heterodontus, including the bull-head shark, and the Port 



Fio. 256. Mackerel-shark or Salmon-shark, Lamna comubica (Gmelin). 

 Santa Barbara, Cal. 



Jackson shark, still persists. The others diverge to form the 

 three chief groups of the cat-sharks (Scyliorhinus, etc.), 

 the mackerel-sharks (Lamna, etc.), and the true sharks (Car- 

 charhias, etc.). 



In the second group the vertebrae have their calcareous matter 

 arranged in rings, one or more about the notochordal center. 

 In all these the anal fin is absent, and in the process of speciali- 



