DEVONIAN FISHES OF IOWA 71 



"No species belonging to the class of Cyclostomes has been 

 found fossil. We may reason theoretically that the earliest 

 fish-like forms were acraniate or lancelet-like, and that lamprey- 

 like forms would follow these, but this view cannot be substan- 

 tiated from the fossils. Lancelets have no hard parts whatever, 

 and could probably leave no trace in any sedimentary deposit. 

 The lampreys stand between lancelets and sharks. Their teeth 

 and fins might at least occasionally be preserved in the rocks, 

 but no structures certainly known to be such have yet been recog- 

 nized. It is, however, reasonably certain that the modern lam- 

 prey and hagfish are descendants, doubtless degraded and other- 

 wise modified, from species which filled the gap between the 

 earliest chordate animals and the jaw-bearing sharks." * 



Among the earliest and most primitive forms of fish-life with 

 which Palaeontology acquaints us, there appears in the Middle 

 and Upper Silurian, and continues thenceforth throughout the 

 Devonian, a curious group of Craniates whose organization 

 stands in sharp relief to that of fishes proper, whose more pre- 

 cise relations are still considered doubtful, and whose origin is 

 involved in complete obscurity. This group is commonly known 

 under Cope's title of Ostracophores, or Ostracoderms. Appear- 

 ing suddenly and unheralded, and passing away at the close of 

 the Devonian without leaving descendants, we can only specu- 

 late in regard to the ancestry of these creatures; yet the in- 

 ference seems warranted that they took their rise from Pro- 

 tochordates at about the same time as primitive Elasmobranchs, 

 and diverged in a different direction. Their adaptive energy 

 was expended chiefly in the development and elaboration of a 

 hard external skeleton, and their progressive modifications 

 stopped short of acquiring completely formed jaws, of arches 

 for the support of paired limbs, and of the ordinary type of fish 

 fins. Indeed, appendages of any kind do not occur except within 

 the limits of a single family,! the Asterolepidae, where jointed 



* Jordan, D. S., Guide to the Study of Fishes, 1905, vol. 1, p. 487. 



t On the alleged occurrence of several pairs of appendages in the Cephalaspidae 

 and Tremataspidae, see the following papers by Professor Patten in the American 

 Naturalist for 1902 and 1903: On the Structure and Classification of the Tremat- 

 aspidae. 36, pp. 379-393. — On the Appendages of the Tremataspidae. 37, pp. 

 223-242 (with critique, p. 573). — On the Structure of the Pteraspidae and Cephal- 

 aspidae. 37, pp. 827-859. 



