THE ORIGIN OF MAN 35 



tails may be necessary. The tail presents three 

 general types. (1) The vertebral column may 

 be straight at its posterior end, continue in con- 

 stantly diminishing joints nearly to the end of 

 the animal, and there may be surrounded by a 

 symmetrical vertical tail fin. (2) In others, the 

 vertebral column bends upward toward the dor- 

 sal side and terminates with diminishing joints 

 in the upper lobe of the dorsal fin. (3) The 

 modern bony fishes also have a symmetric tail, 

 but, while the vertebral column here again bends 

 upward, it ends rather abruptly and from the 

 terminal joints the fin rays develop in such a 

 way as to form a symmetric tail. The first type 

 is the most primitive and was very common 

 among the Devonian fishes and is known as the 

 diphycercal or symmetrical tail. The second 

 type is most common in the Paleozoic fishes and 

 is known as the heterocercal tail because of the 

 inequality of the lobes. While the third type is 

 most common among the modem fishes and is 



