THE GEOLOGICAL RECORD 



201 



very bottom of the vertebrate series and was probably derived from 

 some primitive chordate ancestor, not unlike Amphioxus, which 

 had no hard structures to be handed down to us in a fossil 

 condition. It is true that the ostracoderms bear a curious 

 resemblance to certain extinct contemporaneous arthropods, the 

 Eurypteridse (Fig. 137), 

 but this resemblance is 

 probably merely super- 

 ficial and due to conver- 

 gence owing to similar 

 conditions of life; it 

 cajnnot ha accepted as 

 evidence of the origin of 

 vertebrates from arthro- 

 podan ancestors, which, 

 on other grounds, is ex- 

 tremely improbable. 



The most primitive 

 group of true jaw-bearing 

 fishes is that of the Elas- 

 mobranchii, including the 

 existing sharks, dog-fishes, 

 skates and rays. These 

 have well developed paired 

 fins and a dermal armour 

 of scales or denticles, but 

 their primitive character 

 is shown by the fact 

 that the internal skeleton 

 remains cartilaginous 

 throughout life. Frag- 



FIG. 137. Pttrygotus osiliensis, an Upper 

 Silurian Eurypterid, dorsal view, re- 

 duced. (From Wood's " Palaeontology," 

 after Schmidt.) 



mentary indications of 



elasmobranchs occur 



already in the Upper 



Silurian, but the first satisfactory specimens have so far been met 



with in rocks of Lower Devonian age (Old Red Sandstone). A- 



carboniferous form, Acanthodes, is represented in Fig. 188. 



The remarkable Dipnoi, feebly represented by the mud- 

 fishes or lung-fishes of the present day the Australian Neo- 

 ceratodus (Fig.* 110), the African Protopterus and the South 

 American Lepidosiren seem to have arisen during the 



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