FOSSIL SHELLS. 



to that in which it disappears, is distinguished by the presence of 

 a peculiar species. Of 222 species of Ammonites 17 belong to the 

 oldest fossiliferous rocks, 7 to the carboniferous system, 15 to the 

 new red sandstone, 137 to the oolite, and 47 to the chalk. 



68. Among the organic remains characteristic of strata or groups 

 of strata, the following examples may be mentioned : Fossil Cepha- 

 lopodes are exceedingly numerous in the paleozoic group ; but of 

 all the genera hitherto discovered, one only, that of the nautilus, 

 has come down to the present times. 



These fossils appear in great numbers in the lower strata of the 

 secondary rocks, aie few in the lias and oolite groups, re-appear in 

 great numbers again in the cretaceous, and nearly disappear from 

 the tertiary rocks. 



The examples are so numerous, and preserved in such per- 

 fection, that it is difficult to select any in preference to another, 

 as illustrations of their forms. The nautilus, the only surviving 

 genus of the Tentaculiferous Cephalopodes in the first periods of 

 animal life, had nearly the form which it still retains. 



Fig. 13. Cyprtea Elegans. 



Fig. 12. Murelnsonia Bigrauulosa. 



69. Fossil Gasteropodes are, like the Cephalopodes, extremely 

 numerous. The terrestrial and fluvial genera have in general 

 appeared for the first time in the Tertiary period ; but marine 



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