MARINE VERTEBEATES 315 



a girdle. This girdle corresponds with the shoulder-blade of higher 

 animals, and gives direct attachment to the rays of the fin, which 

 may be regarded as the equivalent of the fingers, and thus there is 

 no part of the limb corresponding with the arm. The pelvic fins 

 also are frequently jointed to a pelvic girdle or hip, but this is a 

 very rudimental structure, or is even entirely absent in some species. 

 The rays of the caudal fin articulate with the extremity of the 

 backbone, but this portion of the fish's anatomy undergoes such 

 remarkable changes that we must devote a few words to it. It is 

 probably well known to our readers that the tails of fishes exhibit 

 three distinct forms. The first of these is a simple fringe formed 

 by the union of unaltered dorsal and ventral fins ; the second is 



Fio. 228. THE SKELETON OF A FISH (PERCH) 

 <J, dorsal fin ; p, pectoral fin ; t>, pelvic fin ; t, tail fin ; a, anal fin 



the unsymmetrical or unequally lobed tail so characteristic of 

 sharks, dogfishes, and rays ; and the third is the broad symmetrical 

 tail fin, often distinctly forked or bi-lobed, such as we meet with in 

 the majority of our bony fishes. These three kinds are known 

 respectively as the cLiphy cereal, heterocercal, and the homocercal 

 tails. 



Now, it is an interesting fact that the most ancient fishes of our 

 globe possessed tails of the first type ; and that these gradually 

 gave place to the heterocercal form ; while the higher fishes of the 

 present day nearly all possess the homocercal tail. Thus as time 

 advanced the heterocercal tail was gradually evolved from the 

 diphy cereal, and the homocercal from the heterocercal. 



