HOW THE FISHES LEARNED TO SWIM — HEINTZ 



233 



More interesting is the development of the second group of the 

 Pteraspidomorphi, the so-called Heterostraci, which are known from 

 the upper Silurian to the Devonian (fig. 1). 



5ub.0rd. Cyathaspicla. 



Figure 6. — Different representatives of the suborder CyathaspiJa (after Kiaer). A, 

 Anglaspis from Spitzbergen ; from the side, from above, and cross-section of anterior 

 part of body and of tlie tail. B, Poraspis trom Spitzbergen ; from the side, and cross- 

 section of the shields. C, Ctenaspis from Spitzbergen ; head shields from above, and in 

 frontal view. D, Cyathaspis, cephalothorax from above. 



The oldest (Cyathaspids), which have been thoroughly described 

 by Kiaer, resemble in their shape the oldest Ost^ostraci (fig. 6). 

 The head and the foremost part of the body are also covered by a 

 shell, which, however, does not consist of a continuous armature, as 

 in Tretnataspis^ but of two large plates, one dorsal and one ventral. 



