THE DEVONIAN PERIOD 



139 



Silurian. Their skeletons are wholly cartilaginous, the only hard 

 parts being the teeth and fin spines which are commonly preserved 



Fig. 81 



A Paleozoic (early Mississippian) Selachian or Shark, Cladoselache 



fyleri. (Restored by Dean.) 



as fossils. The arrangement of separate gill slits in the throat 

 wall is a more eel-like than fish-like feature. Simple, paired fins 

 are present, but 

 scales or plates 

 are absent. They 

 were common in 

 the Devonian 

 seas, and also 

 probably in lakes 

 and lagoons. Fig. 

 81 exhibits a typ- 

 ical Paleozoic 

 species which is 

 very similar to 

 living forms. 



Dipnoans are 

 remarkable in 

 being able to 

 breathe both in 

 water and air, 

 since they have 

 both gills and 

 lungs, the air- 

 bladder being 

 more or less used 

 as a lung. They were abundant during Devonian time. Fig. 82a 

 shows a common Devonian species which is remarkably similar 

 to the modern Ceratodus. Note the paddle-shaped paired fins, 



Fig. 82 

 Devonian Fishes: a, Dipnoan, Dipterus valenciennesi, 

 (restored by Traquair); b, Arthrodiran, Coccosteus 

 decipiens (restored by Woodward); c, Ganoid, 

 Osteolepis (restored by Nicholson). 



