330 



ORDERS OF FISHES. 



SUB-ORDER E. ACANTHODID^E. Scales exceedingly small, 

 shagreen-like ; the front of each fin provided with a strong 

 spine, simply implanted in the flesh; no distinctly ossified 

 cranial bones ; no operculum ; tail heterocercal. In their fin- 

 spines, and in some other points, the Acanthodid& approximate 

 closely to the Elasmobranchii ; but they are generally regarded 

 as an order of the Ganoidei. The Acanthodidcs are mainly 

 Devonian, but some forms occur in the Carboniferous Rocks, 

 and a species from the Permian Rocks has been doubtfully 

 referred here. Acanthodes (fig. 291) has a single dorsal fin, and 



Fig. 291. i. Acanthodes Mitchelli. 2. Clitnatius scutigtr. 

 3. Diplacantkus gracilis. Devonian. 



is represented in both Devonian and Carboniferous deposits. 

 Diplacanthus (fig. 291, 3) has two dorsal fins, and is exclusively 

 confined to the Devonian Rocks. 



SUB-ORDER F. OSTRACOSTEI. The Ganoids of this sub- 

 order are characterised by having the head, and generally the 

 anterior portion of the trunk as well, encased in a strong ar- 

 mour 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 noto- 

 chord is persistent, the peripheral elements of the vertebrae 

 are ossified. The fishes belonging to this section 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. 



