194 



THE WORLD OF LIFE 



CHAP. 



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

 vertical plates on the sides of the body, as in the species 

 here shown, 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 representing the four limbs of reptiles and 

 mammals. The earliest 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.) 



Fig. 44. — Homocercal (equal-lobed) Tail. 



Modern type of true fishes. 



The older types persist in some of the lower forms. (B.M. Guide.) 



This advance in development is well indicated by the jfc^ 

 gradual changes in the tail, as shown in the accompanying 

 figures (42-44). The upper one is the oldest ; but it soon 

 became modified into the second, which in various modifica- 



