486 



GEOLOGY. 



of being initial vertebrates, they formed the climax and almost the 

 end of their own strange race, for they practically disappeared with 

 this period. Their disappearance is not surprising in view of the 

 development of powerful fishes, for the ostracoderms were obviously 

 not a masterful race. Besides being small, their forms were clumsy 

 in the main, their locomotive organs lacked flexibility and efficiency 

 of form, and their mouth-parts were weak, and poorly situated for 

 aggressive service, as were also their eyes. They probably plowed 

 the soft bottoms of the sluggish waters, half-buried in the mud, above 

 which little beside their peculiarly placed eyes 

 and the backs of their plated bucklers were 

 habitually exposed. They flourished in the 

 fresh waters of the eastern Canadian provinces, 

 as also especially in England, Scotland, Wales, 

 and northwestern Russia. With little doubt 

 there was a migratory zone between these re- 

 gions. While the ostracoderms are sometimes 

 reported as occurring in beds with marine fos- 

 sils, there is little evidence that they were true 

 dwellers of the open sea. 



The cyclostomes.— Another strange class of 

 organisms related to the fishes, but not true 

 fish, was represented by the singular little 

 lamprey-like form, Palceospondylus (Fig. 218), 

 which has awakened much interest because of 

 the suggestion that it was really an ancestral 

 lamprey, a suggestion supported by many of 

 its characters; but opinions differ as to the 

 real nature of this relationship. However this 

 may be, the vertebrate idea is represented in 

 Scotland. (After Dean.) g re at simplicity; a slender column of vertebrae, 

 modified at one end into a head and finned at the other for a tail, 

 without ribs, or paired fins, or any suggestion of paired limbs, 

 make up the known structure. It seems to represent a quite primi- 

 tive type of the vertebrates. The little creature was scarcely two 

 inches long, and so far as known was represented by a single species 

 only, found at Achanarras, north Scotland. 



The wide deployment of the fishes. — The true fishes found in the 



Fig. 218. — Palceospondylus 

 gunni, restored by Tra- 

 quair; from the Old Red 

 Sandstone, Caithness, 



