254 



ZOOLOGY 



SECT. 



ORDER 1. HETEEOSTRACI. 



This order includes four families, the Pteraspidce, the Ccelolepidce, the 

 Drepanaspida:, and the Psammosteidce. Of the first Pteraspis (Fig. 927) may 



FIG. 927. Pteraspis rostrata (Devonian). (From the Brit. Mus. Cat. of Fossil Fishes.) 



be taken as an example. The body is elongated, and divided into an anterior 



region, representing the head and fore-part of the trunk, and covered by strong 



calcified plates or scutes, and a posterior or caudal region covered by rhornboidal 



scales. In the anterior region there 



are seven scutes above, constituting 



the dorsal shield, while below there is 



a single ventral shield. The dorsal 



shield is produced into a rostrum, 



and is hollowed by a pair of lateral 



orbits, between which is a pit, on the 



inner surface of the shield, probably 



marking the position of the pineal 



Fia. 928. Restored outline of Lanarkia 

 spinosa, in the position in which it 

 occurs as a fossil, the head being flat- 

 tened and the tail twisted round so as 

 to appear in profile, tin each side a 

 much enlarged dermal denticle is 

 shown. (From the Cambridge Natural 

 History, after Traquair.) 



FIG. 929. Restored outline of the dorsal sur- 

 face of Drepanaspis gemundenensis. 



The tail appears in profile, m. d. median 

 dorsal plate ; p. I. postero-lateral plate ; 

 r. rostral plates. (From the Cambridge 

 Natural History, after Traquair.) 



body. The scutes contain no lacunae or canaliculi, and have not, therefore, 

 the structure of bone : they are lined by a nacreous -layer, and are covered 



