104 LIFE ON THE EARTH. 



Two other orders have horny scales, neither bony 

 nor enamelled viz. Cycloid and Ctenoid Fishes; 

 and these, beginning their courses with the Mesozoic 

 Strata, constitute by far the largest portion of the 

 existing race of fishes. Here again we perceive the 

 strong contrast between the Palaeozoic and Neozoic 

 inhabitants. 



Existing marine Fishes have tails formed on 

 different models, as the forked tail of the Salmon, the 

 rounded tail of the Wrasse, the elongate pointed 

 tail of the Conger, arid the unequally lobed tail of the 

 Shark. The three former tails may be called regular 

 or symmetrical, the latter unsymmetrical ; or if we 

 please, the fishes possessing the former may be called 

 Homocercal, while the latter may be called Hetero- 

 cercal. In existing nature the Homocercal fishes are 

 by far the most numerous; they are plentiful in 

 Caenozoic and Mesozoic Strata, but they are unknown 

 in the Palaeozoic rocks, where Heterocercal fishes 

 alone occur. This is the more remarkable because 

 both in the Palaeozoic and Mesozoic rocks Ganoid 

 fishes abound ; still Palaeozoic races are all hetero- 

 cercal, Mesozoic are mostly homocercal. In all pe- 

 riods Placoid fishes were heterocercal. 



The group of Placoid Fishes is the only one 

 which has existed through all geological periods to 

 the present day ; it includes a large range and gra- 



