134 



THE GEOLOGIST. 



Austria. This zone includes twenty-five species, differing 

 from those in the two former. 



Having again passed over several beds in the ascending 

 order^ a fourth zone of Rudista is found in the basin of the 

 Pyrenees. It occupies the right bank of the mouth of the 

 Gironde, and the environs of Bergerac (Dordogne) ; this 

 again includes eight species, distinct from any contained in 

 the three preceding. 



A fifth zone of Rudista is met with in the white chalk of the 

 Paris basin, and in Belgium and Sweden, containing nine species 

 entirely distinct from those of the other zones, and, in addi- 

 tion, these belong to the genus Crania^ which is unknown in 

 the rest of the cretaceous system. Having reviewed these 

 facts, and considered the geological basins in their diiferent 

 relations to each other, and the species of Rudista therein 

 contained, M. d^Orbigny draws the following inferences, 

 which he considers very important, in their zoological rela- 

 tion, as well as being the means of detecting and classifying 

 the several geological epochs. 



1 . — The Rudista, from not being disseminated throughout 

 the mass, but, on the contrary, forming successive and limited 

 deposits, are the best means of determining the limits of the 

 beds. 



2. - — These distinct zones, included within the limits of the 

 same basin, and in a succession of beds, which have suffered 

 no dislocation, prove that no great commotions were required 

 to affect the assemblage of different fauna into one locality, 

 but that, doubtless, there was a regular succession of forms, 

 one fauna being successively replaced by another. 



3. — The Rudista having appeared on the earth at five dis- 

 tinct times during the cretaceous period, each time under 

 different forms, without any zoological blending of species, 

 or transport of individuals from one geological zone to the 



