HOW THE FISHES LEAENED TO SWIM HEINTZ 



237 



on the other hand, was not torpedo-shaped but entirely flat, of a typ- 

 ically benthonic character. A second group, the Anaspids, had tor- 

 pedo-shaped bodies, but were, on the other hand, w^ithout any highly 

 developed organs of steering and equilibrium. 



When we now turn our attention to the class Pisces, or the true 

 fishes (fig. 1), then it is particularly the so-called Placodenns which 

 are of interest. With the Elasmobranchs,^ they belong to the oldest- 

 known true fishes, and their first remains are found at the transition 

 from the Silurian to the Devonian. These oldest forms belong to 



Lin. Rl Arthrodira. 



Coccosteua. 



FicuRE 8. — Different .Arthrodira (after Broili and Hointz). A, Acantluispis from Spltz- 

 bergen ; (1) from above, (2) from the side, (3) body shield seen from the front, (4) 

 cross-section of a lateral spine and the side of the body shield. B, Acanthaspis from 

 Germany ; with completely preserved posterior part of trunk. C and D, two different 

 types of body shields of Acanthaspids from Spitzbergen. E. Coccosteus, an Arthrodlre 

 from the Middle Devonian ; internal skeleton strongly calcifld. 



the subclass Arthrodira and the order Acanthaspida, which has be- 

 come particularly well known through the discoveries in the Lower 

 Devonian of Spitzbergen. Everything seems to indicate that this 

 oi-der must be regarded as the central systematic unit among the 

 Arthrodires, from which the different other families have sprung. 



The head and tlie foremost part of the body of the Acanthaspids 

 (fig. 8, A, B) were covered by a solid shield as in all other Arthrodira. 

 But whereas we find the shield of the head and body in the Ostraco- 

 derms united into a single shell, the cephalothorax, the shields of the 



^According to Stensio's investigations, the Placoderms must be regarded as belonging to 

 the Elasmobranchs. 



