202 THE STORY OF FISH LIFE. 



that which gave rise to the sharks. The 

 study of the Coelolepidae has thrown a flood of 

 light upon some otherwise unintelligible fish 

 remains found in the old red sandstone, in the 

 form of skin-plates. These skin-plates prove to 

 be made up by the fusion of shagreen denticles 

 resembling those of the Coelacanths. This dis- 

 covery is one of great importance, for it establishes 

 a connecting-link between the creatures who wore 

 this ancient armour-plate, and who have been 

 christened by the generic name of Psammosteus, 

 and the Coelolepidse on the one hand, and the 

 enigmatical Heterostraci, on the other. For by a 

 precisely similar fusion of denticles the head- 

 armour of these curious forms was probably 

 derived. Indeed it is believed that traces of 

 this fusion are obvious in the concentric lines 

 which mark the separate elements making up 

 the armour, which we have already described 

 (p. 198). 



The Ccelolepidse may be included both as 

 puzzles and patriarchs. So also may the re- 

 markable fossil-forms known as the Arthrodira. 

 The fishes of this group attained enormous size. 

 The head and anterior end of the body were 

 heavily armoured with bony plates. Between 

 the head-shield and the dorsal-shield of the trunk 

 immediately behind, a very perfect and elaborate 

 joint was formed hence the name Arthrodira 

 joint-necked. This is a feature unique among 

 fishes. One of the largest of the group was the 

 Dinichihys of the Upper Devonian, Ohio, U.S.A. 

 The Arthrodira are generally held to be ancient 

 lung-fish. No trace has yet been found of 



