DEVONIAN FISHES OF IOWA 59 



Ostracophores, though forming the dominant feature of Silu- 

 rian fish life, and making, as we have seen, considerable progress 

 in the development of an external skeleton, were yet accompanied 

 in the Upper Silurian by creatures which surpassed them in 

 grade; and these vertebrates, by reason of having completed 

 their jaws and acquired a pair of lateral fin folds, are entitled 

 to rank as true fishes. These oldest remains of typical fishes 

 evidently belong very near to that primordial stock from which 

 is descended the great group of Elasmobranchs, a group repre- 

 sented at the present day by sharks and skates, and whose early 

 offshoots are commonly held to be ancestral to all higher types of 

 ordinary fishes. No connection can be traced between the earliest 

 known Elasmobranchs and Ostracophores, yet it is interesting 

 to note that dermal armor originates among them in precisely 

 the same fashion as already noted among primitive Coelolepids. 

 That is to say, these very old Elasmobranchs, which are called 

 Acanthodians after the name of the first described genus, re- 

 semble the oldest fossilized Ostracophores in having the body 

 completely covered by small, hard skin-granules. Not only did 

 the armor begin among Acanthodians in the same way as in the 

 most primitive fossil chordates, but there was also occasional 

 fusion of the skin-granules into plates where rigidity was pos- 

 sible or necessary. This tendency is sometimes carried even to 

 disadvantageous extremes, as in the case of the unduly stiffened 

 and cumbersomely armed paired fins. Evolutionary progress 

 in the direction of improved swimming-organs is very clearly 

 indicated by Acanthodian fin-structures, the general trend of 

 development being succinctly stated by Smith Woodward in the 

 following language : 



"These very old Acanthodians are known because they are 

 completely covered by small, hard skin-granules like those of the 

 oldest fossilized Ostracoderms. ... A few of the granules 

 fused together at the front edge of the median fins above and 

 below the body, thus forming cut -waters or ' ' spines ' ' ; and as a 

 double series of exactly similar "spines" occurs along the lower 

 border of the abdomen where the two pairs of fins are found in 

 later fishes, it is reasonable to infer that these are likewise the 

 stiffened front edges of fins. In other words, paired fins were 

 not originally restricted to two pairs, but formed a double series 



