DEVONIAN ERA. 



attention. " In all recent fish, with the exception of the shark 

 family, the sturgeon, and the bony pike, the vertebral column 

 terminates at the point where the caudal fin is given off, and 



FIG. 25. 



A, Homocercal Tail; B, Heterocercal Tail. 



this fin is expanded above and below the body, forming what 

 is called a homocercal tail. In all those, without exception, 

 which have been found in strata of the Palaeozoic periods [pla- 

 coids and ganoids], the caudal fin is heterocercal, being formed 

 of two unequal branches, the upper one expanded immediately 

 from the vertebral column, while the lower one is given off at 

 a point some distance from the extremity." 1 It is a remarkable 

 fact, that this one-sided tail is a peculiarity in the more perfect 

 fishes (as the salmon) at a certain stage in their embryonic 

 history ; as is also the inferior position of the mouth, peculiar 

 to the early fishes. Moreover, in the early periods of embryonic 

 life, there is no vertebral column, this organ being represented 

 in embryos by a gelatinous cord, called the dorsal cord or noto- 

 chord, which in maturity disappears as the vertebrae are formed 



1 Ansted's Geology, i. 185. 



