200 THE STORY OF FISH LIFE. 



known specimens of which have been found in 

 Forfarshire and Herefordshire. This fossil 

 presents many features of peculiar interest. 

 One of the most important concerns its general 

 form, which is curiously like that of the old 

 trilobites, the ancient Crustacea amongst which 

 it lived, arid which for some inexplicable reason 

 it seems to have mimicked. As will be seen in 

 fig. 16 B, the head-shield is of considerable size, 

 and in some species was produced backwards into- 

 two bony spines, and these again bore spines, 

 which it is surmised were used in progression. 

 The body was ensheathed in numerous hard plates, 

 disposed in bands round the body. Some of 

 these plates rise up in the middle line of the 

 back to form a dorsal fin. By the way, the need 

 for a dorsal fin seems to be a very real one, 

 judging by the totally different structures which 

 have been made to serve this purpose. The 

 spine in Pteraspis, the arched scales of Cepha- 

 laspis, cartilaginous, horny and bony rays in the 

 higher fishes, and fatty tissue in the aquatic 

 mammals the whales, the porpoises, and the 

 dolphins. A further interesting feature of 

 Cephalaspis is the possession of a pair of flap-like 

 structures behind the head-shield, which it has 

 been suggested, represent not fins but gills. 



The Antiarcha represent the most highly 

 specialised of these ancient puzzles. The genus 

 Pterichthys contains the typical species. The 

 armour-plating of the head and trunk was very 

 complex and perfect, the separate plates over- 

 lapped one another (fig. 16 (7). Another feature 

 of these plates is the series of shallow grooves. 



