146 



ORDERS OF FISHES. 



been described. Acanthodes has a single dorsal fin, and is 

 represented in both Devonian and Carboniferous deposits. 

 Cheiracantlms (fig. 511, 1), of the Old Eed Sandstone, 

 is very similar to Acanthodes, but the single dorsal fin is 

 placed in front of the anal. Diplacantlms (fig. 511, 3) has 

 two dorsal fins, and is exclusively confined to the Devonian 

 rocks. 



Fig. 511. — 1, Cheiracanthus Murchisoni ; 2, Cliinatius scutiger ; 3, Diplacantlms 

 aracilis. Devonian. 



Sub-order F. Ostracostei. — The Ganoids of this sub- 

 order, commonly known as " Placoderms," are characterised 

 by having the head, and generally the anterior portion of the 

 trunk as well, encased in a strong armour composed of 

 numerous large ganoid plates, immovably united to one 

 another. The posterior extremity of the body is more or 

 less completely unprotected ; and whilst the notochord is 

 persistent, the peripheral elements of the vertebrae may be 

 ossified. The fishes belonging to this section — if the 

 piscine nature of the " Conodonts " be denied — are the 

 most ancient of their class, commencing in the Upper 

 Silurian rocks. They extend through the Devonian series, 

 but are not known to have survived into the Carboniferous 

 period. They have generally been placed amongst the 

 G-anoids ; but Professor Huxley has pointed out that they 

 present, many of them, features by which they approximate 



