250 A "MARINE FAUNA" DEFINED. 



may be found living at one time in the same sea is 

 not sufficient. Many bygone assemblages of ma- 

 rine animals — palaeozoic, oolitic, cretaceous, and 

 nummulitic — have in turn tenanted the waters of 

 the Atlantic depression ; and each has extended 

 across the same zones in latitude as does our exist- 

 ing European fauna. If, with respect to the pre- 

 sent, we limit a fauna to such forms as co-exist, no 

 comparison with the past can be made ; the two 

 assemblages represent in one case a definite, in the 

 other an indefinite, portion of time. 



In those great assemblages known to the palaeon- 

 tologist as " the fauna of the Cretaceous period," 

 or of the " Nummulitic period," are comprised 

 forms of which we know that they did not all co- 

 exist ; and, further, that each period was marked in 

 every latitude by the constant in-coming and out- 

 going of distinct species. We can ascertain the 

 extent of many an extinct fauna as a whole, from 

 its establishment to its close, though we may never 

 know, except in a very limited degree, what were 

 the relations of its component subdivisions. 



That our existing European marine fauna may 

 have a corresponding value with that of any of the 

 great assemblages of the palaeontologist, it must 

 have a like extension ; it must be dated back, so 

 as to include all those forms which have co-existed 

 since any species now found in the North Atlantic 

 first made its appearance there. 



Vast as are the periods of past time which the 



