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able difference existed between the inhabitants of the 

 waters by which the two formations had been deposited. 

 Hardly a shell is to be found in the blue clay of any genus 

 which has been seen in the chalk. The waters from which 

 the clay and the succeeding crags proceeded, appear to have 

 been chiefly stored with testaceous animals differing from 

 any which had been before in existence. This formation is 

 remarkably characterized by the simple univalves, which in 

 the preceding formations had existed, comparatively, in small 

 numbers, but which now seem to have come into existence 

 in a very predominant superiority. A slight view of the 

 shells of this formation, with occasional reference to those 

 of the preceding formations, it is presumed, will lead to 

 interesting and useful conclusions. 



Of the multilocular univalves, or of the bivalves of 

 complicated structure, occurring so frequently in the earlier 

 formations, perhaps none are to be found in the blue clay 

 except such as have been dislodged, by alluvial action, 

 from the early matrices in which they had been originally 

 imbedded. About thirty-two genera of bivalves had been 

 found in the more ancient strata, and only five or six new 

 genera have been found in the blue clay ; but, on the other 

 hand, the more ancient strata had been found to contain 

 only twelve or fourteen genera of simple turbinated or 

 turriculated univalves ; whilst the blue clay, and its accom- 

 panying sands and crag, have yielded thirty-two genera of 

 turriculated shells; twenty -five of which, with about sixteen 

 other genera not known to have yet existed in a mineralized 

 state, people the waters of the present world with the 

 myriads supplied by their numerous species. 



We hence learn, that almost all the waters from which 

 have proceeded strata containing testaceous remains were 

 inhabited by genera, many of which, though multiplied in 

 numerous species, and in myriads of individuals, have 

 become entirely extinct ; the proofs of their having existed 



