THE GEOLOGICAL RECORD 



209 



and there is often a smaller shield beneath. A separate piece 

 forms a projecting snout. 



The shields of these fishes are often preserved, while the 

 complete body is very rare. 



Another gi'oup (Fig. 41) has the head shield continuous 



Fig. 41. — Ceplalaspis murchisoni. 

 From Old Red Sandstone of Herefordshire. About half nat. szie. 



(B.M. Guide.) 



or in two pieces, while the skin-tubercles are united into vertical 

 plates on the sides of the body, as in the species here sho^vn, 

 while others have two or three rows of plates. 



The highest group of these primitive fishes has the head 

 and fore part of the body covered with large polygonal bony 

 plates. As these died out in the Devonian epoch their place 

 was taken by true fishes, having an ossified skeleton, a movable 

 lower jaw, gill-covers, and pairs of pectoral and anal fins rep- 

 resenting the four limbs of reptiles and mammals. The ear- 

 liest of these were allied to our sharks ; and at each succeeding 

 geological stage a nearer approach was made to the higher 

 types of our modern fishes. 



Class — Pisces 



Fig. 42.— Protocercal Tail. 

 The primitive type of true fishes, having a lower jaw and paired fins. (B.M. Guide.) 



Fig. 43. — Heterocercal (unequal-lobed) Tail. 

 The middle type of true fishes. (B.M. Guide.) 



