The Class Ostracophori 



57S 



in purple roofing-slate in Gmiinden, Germany. This fish, which 

 reaches a length of about two feet, has a broad head, with eyes 

 on its outer margin, with a slender body and heterocercal tail. 

 The head has a broad median plate and smaher polygonal 

 ones. The flaps, supposed to represent the pectoral fins, are 

 here cased in immovable bone. No trace of internal skeleton is 



Fig. 358. — Drepanaspis gmundenensis Schluter. Upper Silurian, Ginunden, 

 Germany. (After Traquair.) 



found by Traquair, who has given the restoration of this species, 

 but the mouth has been outlined. 



The best known of the Heterostracan families is that of 

 Pteraspidcr. In this family the plates of the head are coalesced 

 in a large carpace, the upper part originally formed of seven 

 coalesced pieces. A stout dorsal spine fits into a notch of the 

 carapace. The slender body is covered with small scales and 



Fig. 359. — Pteraspis rostrata Agassiz. Devonian. Family Pteraspidce . 

 (After Nicholson.) 



ends in a heterocercal tail. The dermal sense-organs are well 

 developed. Pteraspis rostrata occurs in the Lower Devonian. 

 Other gen(jra are PaUsaspis and Cyrthaspis. 



Order Osteostraci. — The Osteostraci (oVreoK, bone; ocrrpaKoi^ 

 box) (called Aspidocephali by Rohon) have bone-corpuscles in 

 the shields, and the shield of the back is in one piece without 



