Fig. 1. A species of _Radiolites_ (_R. agariciformis_, of M. D'Orbigny), from the Cretaceous strata of France. This genus is only known in a fossil state; it belongs to the same group of shells (order, _Rudistes_) as the Spherulites and Hippurites: the lower valve is conical, and much larger than the upper, which is slightly convex; it is deeply channelled longitudinally. Fig. 2. Smooth valve of a species of Corbula (_Corbula gallica_, of Lamarck); abundant in some of the Eocene deposits of the Paris basin. Fig. 3. A single valve; the inner surface is shown in the figure, of a remarkable genus of shells (_Crania personata_, of Lamarck), frequently occurring attached to Echinites and other bodies of the white chalk. Fig. 4. A species of _Terebratula_ (_T. diphya_, of Lamarck). The shells of this genus belong to that division of mollusks termed _Brachiopoda_ (arm-feet), from their having internally two spiral fleshy arms developed from the sides of the alimentary orifice. These organs are supported by shelly processes, curiously modified in different genera, which often occur in a fossil state. Although the fossil Terebratulæ are very numerous, the recent species are but few, and are inhabitants of the seas off Australia. They form two natural groups; in the one the shells are smooth, but perforated all over with minute openings or foramina; and these are often filled with a dark substance, which is the carbonized soft parts: in the other division the shells are plicated or furrowed, and are not foraminiferous.[62] The Spirifers, another group of Brachiopoda, have a pair of internal spiral appendages. [Footnote 62: On the structure of shells the reader should consult the admirable papers of Dr. Carpenter, in the British Association Reports.] Fig. 5. _Terebratula coarctata_, of Parkinson. Bradford clay, Wilts. Figs. 6, & 7, show the internal structure of recent Terebratulæ from New Holland. The complicated shelly apophyses which supported the arms are quite perfect. Fig. 8. _Terebratula triquetra_, of Parkinson (_T. diphya_, of Lamarck); another example of the species, fig. 4.