THE ARTHRODIRA. 



389 



plates as in the ostracoderms, but it is greatly reduced in size, and deeply incised 

 on the flanks, thus opening up the peribranchial chamber. It thereby takes on 

 more distinctly the character of a true operculum, and apparently serves solely 

 for the protection of the gills and heart, the digestive and urogenital organs 



Fig. 262, 263, 264. — Side views of the heads of: A, Bothriolepis; B, Coccosteus; and C, Scaumenacia curta, 

 Whiteaves. In the latter, the outer part of the left mandible has been omitted, exposing the large mandibular 

 dental plate and the row of minute teeth on the anterior margin of the mandibles. 



taking up a position farther back, wholly posterior to the respiratory region, as 

 indicated by the large size of the postbranchial section of the trunk, and by the 

 probable location of the cloacal opening. 



In probably all arthrodires, the notochord persisted throughout life with little 

 or no change. Although there are no indications of vertebral centra, we see for 



