THE DEVONIAN AGE. 97 



most ancient and curious of tliese fishes are those 

 singular forms covered with broad plates, of which 

 the Pterasjpis of the Upper Silurian is the herald, and 

 which are represented in the Lower Devonian by 

 several distinct genera. Of these, one of the most 

 curious is the Gephalaspis, or buckler-head, dis- 

 tinguished by its broad flat head, rounded in front 

 and prolonged at the sides into two great spines, 

 which project far beyond the sides of the com- 

 paratively slender body. This fish, it may be men- 

 tioned, is the type of a family highly characteristic 

 of the Lower Devonian, as well as of the Upper 

 Silurian, and all of which are provided with large 

 plate-like cephalic coverings, sometimes with a long 

 snout in front, and, in so far as is known, a com- 

 paratively weak body and tail. They were all pro- 

 bably ground-living creatures, feeding on worms and 

 shell-fishes, and " rooting" for these in the' mud, or 

 burrowing therein for their safety. In these respects 

 they have a most curious analogy to the Trilobites, 

 which in habits they must have greatly resembled, 

 though belonging by their structure to an entirely 

 difierent and much higher class. So close is this 

 resemblance, that their head-shields used to be mis- 

 taken for those of Trilobites. The case is one of 

 those curious analogies which often occur in nature, 

 and which must always be distinguished from the 

 true affinities which rest on structural resemblances. 

 Another group of small fishes, likewise cuirassed in 

 bony armour of plates, may be represented by the 



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