on the Fossils of the older Deposits in the Rhenish Provinces. 335 



considered relatively to the thickness of the beds or duration of the epoch, we shall 

 see, 1st, that the total number of species always increases from below upwards ; 

 2nd, that the progression is very different in each order and in each family, and that 

 this progression is even frequently inverse, either in the different orders of the same 

 class or in the various genera of the same order. If, on the other hand, this deve- 

 lopment of the Palseozoic creation be considered relatively to its horizontal extent, 

 or geographically in relation to space, it will be seen, 1st, that the species which are 

 found in a great number of localities and in very distant countries, are almost always 

 those which have lived during the formation of several successive systems ; 2nd, that the 

 species which belong to a single system are rarely observed at great distances, and that 

 they then constitute local fauna, peculiar to certain countries ; whence it results, that 

 the species really characteristic of a system of beds are so much the less numerous as 

 the system is studied upon a vaster scale *. 



* Our memoir was finished and partly in type when we read with true interest that our conclusions 

 accord with those given by Mr. Phillips in his last work on the Palaeozoic Fossils of Devon and Corn- 

 wall, pp. 154, 162 and 163. 



'2x2 



