214 LOUIS AGASSIZ. [CHAP. ix. 



horny plates covered with a strong, shining enamel, 

 endo-skeleton cartilaginous, in some partly osseous and 

 partly cartilaginous ; (4) the Placoids, with cartilaginous 

 skeleton and skin covered irregularly with enamel 

 plates, sometimes of considerable dimensions, at other 

 times reduced to small points like the prickly, tooth- 

 like tubercles on the skin of rays. 



" This classification allowed easy comparisons and 

 generalizations, and the palseontologic history of the 

 fishes offered results not at all expected and most im- 

 portant. These animals have been completely renewed 

 by successive creations, and whole populations of them 

 have been destroyed to make room for others which were 

 very different Of the four orders indicated above, 

 the Placoids, or cartilaginous fishes, have existed during 

 all the geologic periods, though they have undergone 

 various modifications, most remarkable especially in the 

 teeth. But the other three orders that is, the osseous 

 fishes have somehow replaced one another. Our 

 present seas contain almost altogether Ctenoids and 

 Cycloids, and, except two genera of Ganoids living in 

 rivers of warm countries, these two orders compose all 

 the present fauna of osseous fishes, while, on the con- 

 trary, none existed before the deposit of the chalk, and 

 it would be vain to look in all the preceding epochs for 

 one Ctenoid or one Cycloid ; that is to say, the old seas 

 did not contain a single fish with thin horny scales like 

 our perches or our trout, while in the present fresh 

 waters and seas we find such fishes almost altogether. 



' On the other hand, the Ganoids were most common 

 previous to the Cretaceous epoch, and that order, now 



