THE TRIGGER-FISH 



those which have been commonest since the end of 

 the Mesozoic era, have an ossified skeleton inside, and 

 a covering of thin flexible scales outside. The armoured 

 species are in a minority. During the later Palaeozoic 

 era this was not the case. The fossilized traces of 



the fishes of these days 

 consist especially of 

 armour, which alone 

 made up the principal 

 part of the support- 

 ing structure. Unlike 

 what we find in the fish 

 of today, this struc- 

 ture, instead of being 

 mainly within, was 

 mainly external. A 

 strong, firm cuirass 

 stiffened the body, sur- 

 rounding it like a suit 

 of armour, and this 

 made up the essential 

 part of the skeleton. 

 The remainder, which 

 was cartilaginous, has 

 not come down to us. 



These armoured 

 fishes of the past dis- 

 played a diversity and 

 originality of structure 

 which we do not find 

 in those of this age. 

 Some of them were very large, more than six feet 

 long. One of the best known of them, Pteraspis, 

 had the front part of its body held in a strong 

 shield, made in one piece. Another, Pterichthys, whose 

 armour was made up of broad, welded plates, had 

 pectoral appendages resembling limbs spreading like 

 powerful oars at the side. These species occupied 

 84 



Fig. 16. — Gill-breathing Sea Scorpion 

 [Pterygotus anglicus), a fossil found in 

 Devonian strata of the Palaeozoic era. 

 Some members of this group might 

 be as much as six feet long. 



