DEVONIAN FISHES OF IOWA 53 



Whatever form of vertebrate life may have existed during the 

 Cambrian, and it is reasonable to postulate its existence during 

 that period, no traces of it have been preserved, owing doubtless 

 to the total absence of hard parts. Detached scales, plates and 

 other fragmentary remains of fish-like organisms are known 

 from a few localities of Ordovicic (or Lower Silurian) age in 

 this country, but their very inferior state of preservation pre- 

 vents any reliable conclusions in regard to them. Not until the 

 middle (Niagara in this country) and upper (Ludlow * and 

 Downtown in Great Britain, "Passage Beds" in northern 

 France) divisions of the Silurian do we find at all satisfactorily 

 preserved hard parts of primitive vertebrates, differing in 

 marked degree structurally from ordinary fishes in that they 

 have incompletely formed jaws, no paired fins, and are without 

 calcified endoskeletal parts. Hence, under the name of Ostra- 

 cophores (or Ostracoderms) bestowed by Cope in allusion to 

 their shell-like external covering, they are very properly 

 awarded an inferior position in the scale of piscine evolution. 

 Their lack of a lower jaw articulating with the cranium, a char- 

 acter which they share in common with existing lampreys, sug- 

 gested to Cope the propriety of including both Ostracophores 

 and Marsipobranchs in a separate class, named by him Ag- 

 natha, in contradistinction from Pisces proper. The validity 

 of this distinction appears to be beyond question, and there 

 are other characters ratifying it besides the important ones we 

 have mentioned. Throughout this discussion, therefore, Ostra- 

 cophores will be considered as primitive vertebrates belonging 

 to a lower grade than Fishes proper, and included on that ac- 

 count in a different class, Agnatha, among Protochordates. 

 Their advent slightly preceded that of ordinary Fishes in point 

 of chronological sequence, although both classes probably had 

 a common origin in times anterior to the Ordovician. The ear- 

 liest recognizable indications of Fishes proper appear sparsely 

 (Diplacanth Acanthodians) toward the close of the Silurian. 



*Woodward, A. S., Notes on the Geologv and Fossils of the Ludlow District. 

 Proc. Geol. Assoc, 1904, vol. 18, pp. 429-442.— Hinde, G. J., The Bone-bed in tue 

 Upper Ludlow Formation. Ibid., pp. 443-446. --Leriche, M.. Contribution a l'etude 

 des poissons fossiles du Nord de la France etdes regions voisines. Mern. Soc. Geol. 

 de la France, 1906, vol. 5, Mem. 1, pp. 13-39. 



