HOW THE FISHES LEARNED TO SWIM HEINTZ 



229 



more specialized than in the tadpole. In Tretnataspis there are 

 thre€ conspicuous longitudinal folds which help to maintain equi- 

 librium and a striking heterocercal tail increasing the effectiveness 

 of the forward propulsion. 



Ord. Osteostraci. 



Boreaspis. 



Ho«|ASpis 



CephtkUspis- 



Figure 3. — Different representatives of tlie order Osteostraci (after Patten and Stensio). 

 Tremataspis from Estonia ; from below, above, and cross-section of the body and of the 

 tail. Kiaeraspis from Spitzbergen ; cephalothorax from below, above, from the side, 

 and posterior part in cross-section. Ccphalaspis from England ; from above, from the 

 side, head from below, and in frontal view. Boreaspis from Spitzbergen. Hoeleaspis 

 from Spitzbergen. Long-spincd Cephalaspis from Spitzbergen. 



If we consider the later osteostracs such as Kiaraspis (fig. 3), 

 we will see that evolution has progressed further. In this form, 

 the cephalothorax is more specialized) and divided into a section of 

 the head and a section of the body. On the boundary between these 

 two sections some distinct flat spines have been developed. The 



