Fig. 225.
Radiolites foliaceus, D'Orb. Syn. Sphærulites agariciformis, Blainv. White chalk of France.
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.