THE TRIGGER-FISH 



the seas in great numbers. When we see their re- 

 mains, we cannot help thinking of the soldiers of 

 the Middle Ages, all in coats of mail and helmeted, 

 encased in defensive armour. So, in seas that have 

 now disappeared, most of the fish were similarly 

 covered with plates and shields. But they were not 

 the only fish in the sea. Beside their fossils, we find 

 isolated sharks' teeth, the presence of which is an 

 indication of the purpose of the protective armour. 

 But that is all. Apart from these two groups there is 

 no trace of any other ancient fish population of those 

 early times. 



In company with these fishes dwelt other strange 

 creatures which looked very much like them, so much 

 like them, in fact, that it has been suggested that 

 real affinity existed between them. These were at 

 one time thought to be Crustacea, being provided 

 like them with articulated feet, and a thick jointed 

 carapace investing the body. This group is called 

 the " Gigantostraca ". Their only living representatives 

 are the King Crabs found in warm seas. Its repre- 

 sentatives in long-bygone days were the gill-breathing 

 sea scorpions known as Eurypterus and Pterygotus, 

 with larger, somewhat fish-shaped bodies, and in some 

 of them a pair of limbs was lengthened and spread 

 out rather like pectoral fins. Beside them were hosts 

 of creatures of related structure, but smaller, the 

 famous Trilobites, which seem to have swarmed like 

 the crabs in our own seas, looking like great wood-lice 

 with backs divided into three lobes by two parallel 

 furrows. Doubtless, animals like worms with soft 

 unprotected bodies lived near these creatures which 

 were well provided for on the defensive side, and 

 the diversity we notice today was equally to be seen 

 in their time. But, taking into consideration the 

 number and complexity of the traces they have left 

 behind, traces which we find as fossils, these heavily 

 armoured creatures were proportionately more common 



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