170 VERTEBRATE ZOOLOGY 



of dentine covered with ganoin. Tlie forward part of the body 

 has lateral fin-folds of a very primitive type and the tail is pro- 

 vided with a very primitive heterocercal tail. Elasmobranch char- 

 acters are quite obvious here and it is believed that this group 

 represents a specialized bottom-feeding adaptive radiation from 

 the most primitive shark types of the lower Ordovician or Cam- 

 brian times. 



Another genus that has been placed in this order is Drepanaspis 

 (Fig. 99, F). This creature is much more highly specialized in its 

 exoskeleton than is Lanarkia and furnishes a transition between the 

 latter and the more heavily armed condition of the Pteraspidse, a 

 group that reaches the climax of armature in this order. Drepanaspis 

 is also a broad fiat form with lateral fin-folds and a heterocercal tail. 

 The whole body is covered with a continuous armature of tile-like 

 plates, some of these plates, especially a median dorsal, a median 

 ventral, and paired laterals, being especially conspicuous. The rest 

 of the body is covered with smaller tessellated plates of various 

 sizes. 



In the genus Pteraspis the armor over the cephalothorax is much 

 simplified by dropping out all but the largest plates, and by the de- 

 velopment of a rostral plate. The tail is decidedly fish-like and covered 

 with rhomboidal plates much hke those of the lobe-finned ganoids or 

 the modern gar-pike. A strong dorsal spine is a conspicuous feature 

 of this species. 



Order II. Osteostkachi 



This order resembles the Heterostraci in having the anterior body 

 covered with a solid armor and the tail free to move. They differ 

 from the Heterostraci in having bony plates instead of mere calcifica- 

 tions, in having a dorsal fin, and in having eyes median instead of 

 lateral in position. The carapace reminds one strongly of that of the 

 King Crab (Limulus) and its extinct relatives, but the resemblance is 

 probably merely a superficial one due to similarity of habits. They 

 apparently had "a grovelling bottom-feeding, sluggish habit of life," 

 in contrast with the active predaceous life of their free-living, shark- 

 like ancestors. They played out their string of specialization and 

 became extinct during Devonian times. 



