36 EVOLUTION IN THE PAST 



were covered with little tubercles or prickles of a dentine 

 substance. There was a fin on the back, and a forked tail fin, 

 but apparently there were no paired fins as in the case of all 

 true fishes (Thelodus). In some species the skin-prickles stood 

 out like little spines (Lanarkia). 



Other forms, similarly finned, were spindle-shaped like 

 most living fishes. The tubercles on the body had been fused 

 into well-defined scales ; but those of the head had not under- 

 gone the process. These creatures, unlike the flat -headed 

 forms, possessed on the under-part of the bod}' a row of 

 spiny scutes (Birkenia). Other allied forms had a few rod-like 

 structures at the back of the head, and were liberally supplied 

 with scutes along the ventral region ; but they do not appear 

 to have had the other parts of the body protected (Lasanius). 



Yet other odd-looking little forms had the head and the 

 anterior part of the body encased in shields consisting of small 

 shelly plates. The rest of the body — to judge from more 

 complete remains found in the next Period — was protected by 

 enamelled or ganoid scales {Cyathaspis, Palceaspis, Pteraspis). 

 A few forms were shielded in the region of the head only. 

 But this armour, less in quantity, was of better quality, for it 

 contained a certain amount of bony material (Tremataspis, 

 Cephalaspis). The appearance of bone is notable : for bone- 

 making ceUs were rare in those days. " Backbones " at best 

 were as yet only of a gristly, cartilaginous substance. 



These various fish-like forms — known as Ostracoderms — 

 were probably of somewhat inactive habits, and seldom rose 

 much above the sea-floor. They did not, therefore, greatly 

 disturb the prevailing quiescence. 

 SHARKS True fishes were also in Silurian seas. Certain small fin- 

 spines have been discovered, resembling in character the 

 spines that protect the dorsal fins of Port Jackson sharks 

 (Onchus). If these creatures fed on molluscs, their tables were 

 amply spread. There is also evidence of small sharks of 

 another kind (Climatius) ; and small sharks, there is little 

 doubt, were the first true fishes that ever swam the seas. 

 These small Silurian forms may have been derived from 

 creatures much of the type of the prickly-skinned, unshielded 

 ostracoderms (Thelodus). The shielded ostracoderms held 



