Geological Arrangement of British Fossil Shells, 37 



From data so ample, we can now obtain accurate notions of 

 the geological position of certain groups of shells ; of the rela- 

 tive proportion of the different classes, and of how far theii 

 structure has been influenced by different degrees of antiquity. 

 To render this analysis more complete, it may be useful to 

 make three principal divisions of the formations containing 

 organic remains. The first is the most ancient series ; the 

 second, or middle, division contains the remainder of the 

 secondary strata; and the third consists of the tertiary 

 deposits. 



The first, which is also the lowest or most ancient division, may be 

 subdivided into two series of formations. 



1. Carboniferous order 

 of Mr. Coneybeare. . 



2. From the carboniferous 

 to the Has, inclusive. 



Species 27 Simple univalves - 9 species. 



34 Simple bivalves - 53 



46 Complicated bivalves 5 



53 Multilocular univalves 50 



140 



97 



The second^ or middle, division, from 

 the lias upwards, includes the entire 

 oolite series, and the strata up to the 

 chalk, inclusive. 



Simple univalves 106 species. 



Simple bivalves 375 



Complicated bivalves 

 Multilocular univalves 139 



620 



The thh'dy or most recent, divi- 



sion, comprises all 

 the chalk, or the 

 tions. 



the beds above 

 tertiary forma- 



Simple univalves 

 Simple bivalves 

 Complicated bivalves 

 Multilocular univalves 



259 species* 

 141 



O 



8 



408 .: 



In treating on the order of arrangement of shells in the 

 several formations, Mr. Parkinson was struck with the fact, 

 that the shells of the most ancient formations exceed, in com- 

 plexity of structure, those in the subsequent strata, and in our 

 present seas. It is in this early creation, also, he observed 

 that those shells are found which possess " that complicated 

 structure, very rarely found in the shells of this day, which 

 enabled their inhabitants to rise and sink with them in the 

 water." Of this latter class are the numerous race of many- 

 chambered univalves, the Nautilites, the Ammonites, and Or- 

 thoceratites ; and of the class of complicated bivalves are the 

 Spirifers, and the genera Pentamerus and Productus. 



Our Table furnishes the means of at once comparing the 

 numbers which existed in each class, during separate periods 

 or geological intervals. 



D S 



