236 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 34 



Paired fins have not been found in the Pteraspids. The tail and 

 the posterior part of the trunk was, on the other hand, more flexible 

 and much more slender than in the Cyathaspids (fig. 6, A). The 

 large, thick Cyathaspid scales have been split into numerous small 

 rhombic scales, which also cover the large hypocercal caudal fin 

 (fig. 7, A, B, E). The cross-section of the tail is a more or less 

 rounded oval, not triangular as in the Cephalaspids (fig. 3) and is 

 without any indications of lateral folds. 



Generally speaking, the Pteraspids must therefore have been 

 bottom forms, which by means of powerful tail strokes lifted 

 themselves from the bottom to glide through the water on their 

 large, wide, lateral spines in the manner of a modern gliding plane. 



Parallel with this development, again corresponding entirely 

 to conditions among the Cephalaspids, we find also that a strong 

 reduction in the thickness of the shields has taken place. In the 

 youngest Pteraspids (from the Lower Devonian) the dermal arma- 

 ture was paper-thin and very light. We thus see that the Cyathas- 

 pid-Pteraspid line of evolution in its adaptations to sw^imming has 

 not reached as far as the Tremataspid-Cephalaspid line which we 

 first described. The latter gradually acquired both organs of equi- 

 librium in the form of spines and lateral and median skin folds, and 

 steering organs in the form of movable paired fins. The Pteraspids 

 did not get further than to the development of lateral gliding spines. 

 No indication of paired fins or lateral fin folds is known in th's 

 group. 



Another line consisting of the Heterostraci, the Drepanaspids and 

 Psammosteids (fig. 7, F) has followed another path of evolution and 

 remain typical fiat bottom forms with short tail, weakly developed 

 lateral spines and no dorso-median projections. We know forms ol 

 this general character from the Lower to the Upper Devonian. Some 

 of them reach a considerable size, over 1 meter in length. 



The recent Cyclostomes, comprising the lampreys and hagfishes, 

 are, as already previously mentioned, closely related to the fossil 

 Ostracoderms. They also have not advanced very far in their 

 adaptation to swimming. They are entirely snakelike (fig. 2, F). 

 winding their way through the water. A median fin fold runs along 

 the posterior part of the back around the tip of the tail and a distance 

 forward along the ventral side. Neither do we find in the Cyclostomes 

 any trace of paired fins. They have special organs of locomotion or 

 of steering, and their organs of equilibrium are only represented by 

 the median fin fold. 



We thus see that the primitive fishlike forms of the order Agnathii 

 (fig. 1) have not attained any specially high adaptation to swimming. 

 Only one single family, the Cephalaspids, had distinctly differenti- 

 ated organs of steering, equilibrium, and propulsion, while the body. 



