d'orbigny on foraminiferje. 71 



ing, and even in the very highest beds of the cretaceous formation the 

 genus Nummulina does not occur. 



In the tertiary the last-mentioned order with numerous genera, all, 

 except two, the Hauerina and the Fabularia, likewise represented 

 in our present seas, appear for the first time. They become more 

 numerous both in genera and species as the beds are more recent, ac- 

 quiring their maximum of development, both generic and specific, in 

 the subapennine beds and the tertiary basin of Vienna. It is also 

 remarkable that species analogous to those now living are only met 

 with in the last stage of the tertiary formations. 



M. d'Orbigny gives this tabular summary of the species peculiar 

 to each geological epoch : — 



Formations . . Carboniferous. Jurassic. Cretaceous. Tertiary. Recent. 

 Genera 1 b M ^Q 68 



Species 1 20 280 450 1000 



In conclusion he observes that the Foraminiferee, few in number 

 and of simple forms, first appear in the carboniferous beds. They 

 become more numerous and complex in the Jurassic strata; still 

 more so in the chalk. In the tertiary formations their diversity and 

 number are greatly increased, and in the actual seas they attain the 

 maximum of their numerical development. The generic forms exhibit 

 a similar progression, many appearing and disappearing with the Ju- 

 rassic, cretaceous and tertiary beds, as if to characterize precisely each 

 grand epoch in the chronological history of the globe. Nothing can 

 be more remarkable than the rapid and increasing succession of genera 

 as the formations approach the present time, or rather, than that pro- 

 gress in perfection of these minute beings, which above all others have 

 in their successive creations proceeded from the simple to the com- 

 plex. The study of the Foraminiferae, continued for twenty-eight 

 years, has produced in M. d'Orbigny the complete conviction that 

 the precise age of a geological formation may be determined from 

 them alone, provided they are compared with the requisite precision. 

 Many genera now live in the seas which are not found in the tertiary 

 beds, yet it is easy to see that the relation between the existing fauna 

 and that of the upper tertiary is far more close than that between the 

 Jurassic and cretaceous faunas, or between the latter and the tertiary. 

 In the present time they have also a peculiar geographical distribu- 

 tion, some genera being found only in warm, others only in cold re- 

 gions, and each species generally confined to a special region. Of the 

 1000 existing species, 575 belong to the torrid, 350 to the temperate, 

 and 7^ to the frigid zone, so that, like the moUusca, they are more 

 numerous and varied in their specific forms as the seas in which they 

 live are warmer. 



[J. N.] 



