DEVONIAN FISHES OF IOWA 57 



been independently derived from some invertebrate progenitor 

 cannot possibly be entertained in tbe light of tlie following facts ; 

 (1) their dermal plates are composed of true bone ;* (2) the head- 

 shield and body armor have a well developed sensory canal sys- 

 tem; and (3), in Pterichthys, at least, there is a tail covered with 

 scales, a membranous dorsal fin, and a genuinely piscine heter- 

 ocercal caudal fin. 



Enlightenment as to the initial stages by which Ostracophores 

 acquired a hard skeleton is furnished by primitive Coelolepid 

 genera, vast quantities of whose granular skin-tubercles occur in 

 the Upper Ludlow bone-bed, and whose complete skeletons have 

 become known during recent years from the Upper Silurian rocks 

 of the south of Scotland. If, now, we are prepared to accept 

 Smith Woodward's contention that the order in which the differ- 

 ent kinds of hard parts were evolved may be reasonably inferred 

 from the order in which they successively predominate, then it 

 becomes an easy matter to understand how these modifications 

 arose. The manner in which these phenomena were progres- 

 sively introduced is thus interpreted by the same author in the 

 following passage. f 



"From numerous well-preserved specimens it is clear that 

 the hard skeletal parts of the Ostracoderms were confined ex- 

 clusively to the skin; and in most of the earliest representatives 

 of the group these hardenings are merely scattered granules or 

 tubercles of limy matter which form a flexible external armour. 

 It is true that each tubercle is beautifully fashioned, with a 

 definite internal structure round a papilla of the skin, like a 

 tubercle from the shagreen of a modern shark; but the armour 

 is essentially a scattered deposit or segregation of superfluous 

 mineral matter in the normally soft tissue, suggesting that the 

 Ostracoderms toward the end of their race had experienced pre- 

 cisely the same affliction as that now experienced by some of the 

 highest mammals in the latter part of their individual life, 

 namely, a kind of "gout". Myriads of the isolated skin-tuber- 

 cles of Thelodus occur in the Upper Ludlow bone-bed, while 

 numerous nearly complete specimens both of this fish (Fig. 1) 

 and Lanarkia (Fig. 2) have been found in the contemporaneous 



*The bone structure of Pteraspis is well described and illustrated by micro- 

 photographic sections in a paper byF. Drevermann, entitled "Ueber Pteraspis dun- 

 ensis," etc. Zeitschr. deutsch. geol. Ges. 1904, 56, pp. 275-289. 



t Woodward, A. S., The Study of Fossil Fishes. Proc. Geol. Assoc. 1906, 19, p. 

 267. 



