Ch. XVL] nummulitic foemations. 229 



an example of the changes which certain species underwent in the succes- 

 sive stages of their existence. It seems that different varieties of the 

 Cardium porulosum are characteristic of different formations. In the 

 Soissonnais this shell acquires but a small volume, and has many pecu- 

 liarities, which disappear in the lowest beds of the calcaire grossier. In 

 these the shell attains its full size, with many distinctive characters, which 

 are again modified in the uppermost beds of the calcaire grossier ; and 

 these last modifications of form are preserved throughout the " upper 

 marine'' (or Upper Eocene) series.* 



Argile plastique (C, Table, p. 222). — At the base of the tertiary sj^stem 

 in France are extensive deposits of sands, with occasional beds of clay 

 used for pottery, and called "argile plastique." Fossil oysters (Ostrea 

 hellovacina) abound in some places, and in others there is a mixture of 

 fluviatile shells, such as Cyrena cuneiformis (fig. 233, p. 220), Melania 

 inquinata (fig. 234), and others, frequently met with in beds occupying 

 the same position in the valley of the Thames. Layers of lignite also 

 accompany the inferior clays and sands. 



Immediately upon the chalk at the bottom of all the tertiary strax? in 

 France there generally is a conglomerate or breccia of rolled and angular 

 chalk-flints, cemented by siliceous sand. These beds appear to be of lit- 

 toral origin, and imply the previous emergence of the chalk, and its waste 

 by denudation. 



Whether the Thanet sands before mentioned (p. 221) are exactly rep- 

 resented in the Paris basin, is still a matter of discussion. 



Wide extent of the nummulitk formation in Europe, Asia, dr. — Wlien 

 I visited Belgium and French Flanders in 1851, with a view of com- 

 paring the tertiary strata of those countries with the English series, I 

 found.that all the beds between the Upper Eocene or Limburg formations, 

 and the Lower Eocene or London clay proper, might be conveniently 

 divided into three sections, distinguished, among other paleontological 

 characters, by three different species of nummulites, JV. variolaria in the 

 upper beds, N. IcBvigata in the middle, and N.planulaia in the lower. 

 After I had adopted this classification, I found, what I had overlooked or 

 forgotten, that the superposition of these three species in the order here 

 assigned to them, had been previously recognized in the North of France, 

 in 1842, by Viscount D'Archiac. The same author, in the valuable 

 monograph recently published by him,f has observed, that a somewhat 

 similar distribution of these and other species in time, prevails veiy 

 widely in the South of France and the Pyrenees, as well as in the Alps 

 and Apennines, and in Istrea, — the lowest nummulitic beds being charac- 

 terized by fewer and smaller species, the middle by a greater number and 

 by those which individually attain the largest dimensions, and the upper- 

 most beds again by small species. 



In the treatise alluded to, M. D'Archiac describes no less than fifty- 

 two species of this genus, and considers that they are all of them char- 



* Coquilles caracteristiques des terrains, 1831. 



f Animaux foss. du groupe nummul. de Tlnde : Paris, 1853. 



