PALEOZOIC TIME — DEVONIAN. 



587 



the old coral reefs, precisely like related kinds in corals of the present day. 

 The related species now living are free-swimming animals in their young 

 state ; the free stage is ended by the animal's coming to rest on the surface 



875-884 



Fig. 875, Head of Dalmanites selenurus ; 876, id. D. regalis ; 877, pygidium of D. aspectans ; 878, id. D. Boothi of 

 the Hamilton beds ; 879, head of Acidaspis callicera ; 880, portion of the pygidium of Acidaspis Eomingeri 

 restored (x ^) ; 881, " head " of Lichas gryps ; 882, id. Lichas hylseus ; 883, posterior extremity of pygidium, 

 restored, of Lichas grandis, from the Schoharie giit ; 884, pygidium of Proetus crassimarginatus, Irom the 

 Corniferous limestone. Hall and Clarke. 



of a living Coral ; and once there, it stays and forms a dwelling cavity lined 

 with shell within the growing Coral, — a case of commensalism, not parasit- 

 ism, it receiving lodging, not board. Similar Barnacles — Palceocreusia 

 Devonica of Hall — were commensals of Devonian Corals, showing that the 

 practice is an ancient one. 



7. Fishes. — Fishes are the only Vertebrates known. The species dis- 

 covered in the Corniferous limestone are : (1) Placoderms ; (2) Dipnoans, or 

 Lung-fishes; (3) Ganoids; (4) Chimgeroids; (5) Selachians, or Elasmobranchs 

 (Sharks). The Placoderms include two species of CepJialaspis, — one from 

 Gaspe (Fig. 885), and the other from Campbellton, New Brunswick (Fig. 886, 



