154 



DEVELOPMENT OF ORGANIC LIFE 



[Ch. IX. 



consider how rich, a molluscous fauna — to say nothing of the 

 crustaceans, sea-urchins, stone -lilies, and corals — have been 



almo st 



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not having yet found 



accompanym 



they were not yet in being, or that they only occupied a 

 limited area. To verify the date of the first appearance of 



more 



reasonably expect, as the first representatives of such types 

 probably originate in one spot only, from which they would 

 spread very slowly over the globe. 



Next 



Silurian comes the Old Red Sandstone or 



Devonian formation, which is so rich in fishes that the 

 number of British species alone described by Agassiz, in 



1844, amounted to sixty-five. Almost all of these belonged 



to the order of Ganoids, and some few only to that of the 

 Placoids, of Agassiz; and it is remarkable that the vast 

 majority of the fossil fish of the succeeding formations, from 

 the Carboniferous to the Oolitic, consist in like manner of 

 Ganoids, a family which, though so rich in genera in the olden 

 times, is of quite exceptional occurrence in the present crea- 

 tion, being confined to the North- American rivers, and those 

 of Africa north of the line. In the chalk, and still more in 

 the tertiary formations, we find the majority of the fish to 

 belong to a great variety of genera of the class called Teleostei 

 because their skeletons are perfectly ossified, which is very 

 rarely the case with the Ganoids of the older rocks. The 

 cartilaginous, persistent nature of the spinal column or 

 notochord, which is not divided into separate vertebrae, is 

 regarded on the whole as a mark of a lower grade, as is 

 the form of the tail called heterocercal, which is almost 

 universal in fish older than the chalk. Nearly all the living 

 fish have equilobed tails, and the heterocercal or inequilobed 

 orm is looked upon by Owen as a retention of the embryonic 



But the 



aiso 



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p 



character, or an instance of arrested development, 

 affinity of the ancient Placoid and Ganoid fishes in the structur 

 of their heart, brain, generative organs, and many other cha- 

 racters to living sharks, as well as to the African Polypt erUS 

 and the bony pike, or Lepidosteus, of America, leads til 



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