Ch. XXVI.] DEVONIAN STRATA IX UNITED STATES, ETC. 



543 



pods to forty-seven Lamellibranchiata. In the antecedent Silurian 

 rocks the relative numbers are still more in favor of the Brachiopods, 

 whereas, in the more modern Carboniferous formation, the proportions 

 are more than reversed, for there are of the Carboniferous Lamelli 

 branchiata 282 species, and only 123 Brachiopoda. 



The reader will of course conclude from what was said at p. 414 

 that all these oolitic species were not living at one and the same time, 

 there having been continual changes going on in the fauna from the 

 period of the lowest to that of the uppermost member of the oolitic 

 series ; but the proportions of the two families of shells may be cor- 

 rectly deduced from the data above given. If we consult the same 

 table to obtain the relative numbers of these same orders of mollusca 

 in the oolites, we find 586 Lamellibranehiata and only sixty-nine 

 Brachiopoda, these last therefore being reduced to nearly an eighth 

 part of the whole bivalve fauna. If we then turn to the actual British 

 seas, we observe that Forbes and Hanley give 220 living species of 

 Laniellibranehiata and only five Brachiopods, the latter being reduced 

 to a forty-fourth part of the whole fauna. As the lamellibranchiate 

 mollusks have an organization of a more complex and higher grade, 

 the fact of their increasing preponderance from the earliest to the 

 latest times has been often cited, and not without reason, as favoring 

 the theory of progressive development. 



Devonian Strata in the United States and Canada. 



In no country hitherto explored is there so complete a series of 

 strata intervening between the Carboniferous and Silurian as in the 

 Lnited. States. This intermediate or Devonian group was first studied 

 in all its details, and with due attention to its fossil remains, by the 

 Government Surveyors of New York. In its geographical extent, 

 that State, taken singly, is about equal in size to Great Britain ; and 

 the geologist has the advantage of finding the Devonian rocks there 

 in a nearly horizontal and undisturbed condition, so that the relative 

 position of each formation can be ascertained with certainty. 



Subdivisions of the New York Devonian Strata, in the Reports of 

 the Government Surveyors. 



Xames of Groups. 





Thickness in Feet. 



1. Catskill group, or Old Red Sandstone, 



- 



2000 



2. Chemung group, 



- 



. 



- 



1500 



3. Portage, ) 



4. Genesee, f 



- 



- 



- 



1000 



5. Tully, - 



- 



. 



. 



15 



6. Hamilton, 



. 



. 



- 



1000 



7. Marcellus, 



. 



. 



- 



50 



8. Corniferous, ) 









50 



9. Onondaga, )' 









10. Schoharie, 



1 - 







10 



11. Cauda-Galli grit, 







12. Oriskany sandstone, - 



- 



- 



5 to 30 



