HOW THE FISHES LEARNED TO SWIM — HEINTZ 



239 



ming. The body is much higher, even though the ventral shield is 

 still rather flat (fig. 9, B). The large spines are strongly reduced 

 and can no longer serve as effective gliding surfaces. The posterior 

 part of the trunk and the tail are, on the other hand, more strongly 

 developed. The scales are entirely reduced and a solid calcified 

 internal skeleton has been developed (fig. 9, E). From its structure 

 we can conclude that the tail was still pointed and tapering, with- 

 out special caudal fins, but Coccosteus, on the other hand, had a 

 strong dorsal fin, an anal fin, and, besides, a set of pelvic ventral 

 fins. In other words, Coccosteus has already developed, in addition 



Un^KI.Arthrodira 



Figure 9. — Gradual evolution of Arthrodira from pure bottom forms to fi-ee-swimming 

 representatives. A, Acanthaspis. B, Coccosteus. C, Dinichthys. D and E, flat, free- 

 swimming forms from German Upper Devonian. F, SoJeiwsteus, an Upper Devonian 

 form in wliich ttie Joint between the hiead and body lias disappeared. (After Gross 

 and Heintz.) 



to the organs of equilibrium in the form of small lateral spines (ho- 

 mologous with the pictoral fins) and median fins, an efficient steering 

 apparatus in the form of paired ventrals. Its entire shape also sug- 

 gests that it must have been a better swimmer than Acanthaspis, in 

 spite of the fact that it was still a characteristically bottom form. 



The further evolution of the swimming Arthrodira goes in the di- 

 rection of a more laterally compressed body and narrower ventral 

 shield, and a more powerful development of the tail (fig. 9, A-E). 

 Parallel with all these changes, corresponding entirely to what took 

 place in the Ostracoderms, we also find a reduction of the thickness 

 of the shield which in the youngest Upper Devonian forms has become 

 paper-thin. 



