_a._ _Radiolites radiosus_, D'Orb. (_Hippurites_, Lamk.) _b._ Opercular valve of same. White chalk of France.] [Illustration: Fig. 225. _Radiolites foliaceus_, D'Orb. Syn. _Sphaerulites agariciformis_, Blainv. White chalk of France.] [Illustration: Fig. 226. _Hippurites organisans_, Desmoulins. Upper chalk:--chalk marl of Pyrenees?[222-A] _a._ Young individual; when full grown they occur in groups adhering laterally to each other. _b._ Upper side of the opercular valve, showing a reticulated structure in those parts, _b_, where the external coating is worn off. _c._ Upper side of the lower and cylindrical valve. _d._ Cast of the interior of the lower conical valve.] The species called _Hippurites organisans_ (fig. 226.) is more abundant than any other in the south of Europe; and the geologist should make himself well acquainted with the cast _d_, which is far more common in many compact marbles of the upper cretaceous period than the shell itself, which has often wholly disappeared. The flutings, or smooth, rounded, longitudinal ribs, representing the form of the interior, are wholly unlike the hippurite itself, and in some individuals, which attain a great size and length, are very conspicuous. Between the region of chalk last mentioned in which Perigueux is situated, and the Pyrenees, the space B intervenes. (See Map, p. 221.) Here the tertiary strata cover, and for the most part conceal, the cretaceous rocks, except in some spots where they have been laid open by the denudation of newer formations. In these places they are seen still preserving the form of a white chalky rock, which is charged in part with grains of green sand. Even as far south as Tercis, on the Adour, near Dax, where I examined them in 1828, the cretaceous rocks retain this character. In that region M. Grateloup has found in them _Ananchytes ovata_ (fig. 212.), and other fossils of the English chalk, together with _Hippurites_. FLORA OF THE CRETACEOUS PERIOD. Although the fossil plants of the Cretaceous era at present known are few in number, the rocks being principally marine, they suffice, according to M. Ad. Brongniart, to show a transition character between the vegetation of the secondary and that of the tertiary formations. The tertiary strata, when compared to the older rocks, are marked by the predominance of _Exogens_, which now constitute three-fourths of the living plants of the globe.[223-A] These exogens are wanting in the secondary strata generally, but in the Cretaceous period they equal in number the _Gymnogens_ (_Coniferae_ and _Cycadeae_) which abounded so much in the preceding Oolitic period, and disappeared before the Eocene rocks were formed.[223-B] The discovery of a tree-fern in the ferruginous sands of the Lower Cretaceous group of the department of Ardennes in France is one of many signs of the contrast of the flora, and doubtless of the climate, of this era with that of the Pliocene and Modern periods.