The Lamp-shells (Brachiopods) have now reached a further stage of the progressive decline, which they have been undergoing
Fig. 193.—A small fragment of Escharina Oceani, of the natural size; and a portion of the same enlarged. Upper Greensand. ever since the close of the Palæozoic period. Though individually not rare, especially in certain minor subdivisions of the series, the number of generic types has now become distinctly diminished, the principal forms belonging to the genera Terebratula, Terebratella (fig. 194), Terebratulina, Rhynchonella, and Crania (fig. 195). In the last mentioned of these, the shell is attached to foreign bodies by the substance of one of the valves (the ventral), whilst the other or free valve is more or less limpet-shaped. All the above-mentioned
Fig. 194.—Terebratella Astieriana. Gault. genera are in existence at the present day; and one species—namely, Terebratulina striata—appears to be undistinguishable from one now living—the Terebratulina caputserpentis.
Whilst the Lamp-shells are slowly declining, the Bivalves (Limellibranchs) are greatly developed, and are amongst the most abundant and characteristic fossils of the Cretaceous period. In the great river-deposit of the Wealden, the Bivalves are forms proper to fresh water, belonging to the existing River-mussels (Unio), Cyrena and Cyclas; but most of the Cretaceous Lamellibranchs are marine. Some of the most abundant and characteristic of these belong to the great family of the Oysters (Ostreidœ). Amongst these are the genera Gryphtœa and Exogyra, both of which we have seen to occur abundantly in the Jurassic; and there are also numerous true Oysters (Ostrea, fig. 196) and Thorny Oysters (Spondylus, fig. 197). The genus Trigonia,
Fig. 195.—Crania Ignabergensis. The left-hand figure shows the perfect shell, attached by its ventral valve to a foreign body; the middle figure shows the exterior of the limpet-shaped dorsal valve; and the right-hand figure represents the interior of the attached valve. White Chalk. so characteristic of the Mesozoic deposits in general, is likewise well represented in the Cretaceous strata. No single genus of
Fig. 196.—Ostrea Couloni. Lower Greensand. Bivalves is, however, so highly characteristic of the Cretaceous period as Inoceramus, a group belonging to the family of the Pearl-mussels (Aviculidœ). The shells of this genus (fig. 198) have the valves unequal in size, the larger valve often being much twisted, and both valves being marked with radiating ribs or concentric furrows. The hinge-line is long and straight, with numerous pits for the attachment of the ligament which serves to open the shell. Some of the Inocerami attain a length of two or three feet, and fragments of the shell are often found perforated by boring Sponges. Another extraordinary family of Bivalves, which is exclusively confined to the Cretaceous rocks, is that of the Hippuritidœ. All the members of this group