Fig. 215.—Turbinolia sulcata, viewed from one side, and from above. Eocene. yielded the remains of the "Sea-pens" (Pennatulidœ) and the branched skeletons of the "Sea-shrubs" (Gorgontidœ).
The Echinoderms are represented principally by Sea-urchins, and demand nothing more than mention. It is to be observed, however, that the great group of the Sea-lilies (Crinoids) is now verging on extinction, and is but very feebly represented.
Amongst the Mollusca, the Polyzoans and Brachiopods also require no special mention, beyond the fact that the latter are greatly reduced in numbers, and belong principally to the existing genera Terebratula and Rhynchonella. The Bivalves (Lamellibranchs) and the Univalves (Gasteropods) are exceedingly numerous, and almost all the principal existing genera are now represented; though less than five percent of the Eocene species are identical with those now living. It is difficult to make any selection from the many Bivalves which are known in deposits of this age; but species of Cardita, Crassatella, Leda, Cyrena, Mactra, Cardium, Psammobia, &c., may be mentioned as very characteristic. The Caradita planicosta here figured (fig. 216) is not only very abundant in the Middle Eocene, but is very widely distributed, ranging from Europe to the Pacific coast of North America. The Univalves of the Eocene are extremely numerous, and generally beautifully preserved. The majority of them belong to that great section of the Gasteropods in which the mouth of the shell is notched or produced into a canal (when the shell is said to be "siphonostomatous")—this section including the carnivorous and most highly-organized groups of the class. Not
Fig. 216.—Cardita planicosta. Middle Eocene. only is this the case, but a large number of the Eocene Univalves belong to types which now attain their maximum of development in the warmer regions of the globe. Thus we find numerous species of Cones (Conus), Volutes (Voluta), Cowries (Cyprœa,
Fig. 217.—Typhis tubifer, a "siphonostomatous" Univalve. Eocene.
Fig. 218.—Cyprœa elegans. Eocene. fig. 218), Olives and Rice-shells (Oliva), Mitre-shells (Mitra), Trumpet-shells (Triton), Auger-shells (Terebra), and Fig-shells (Pyrula). Along with these are many forms of Pleurotoma, Rostellaria, Spindle-shells (Fusus), Dog-whelks (Nassa), Murices, and many round-mouthed ("holostomatous") species, belonging to such genera as Turritella, Nerita, Natica, Scalaria, &c. The genus Cerithium (fig. 219), most of the living forms of which are found in warm regions, inhabiting fresh or brackish waters, undergoes a vast development in the Eocene period, where it is represented by an immense number of specific forms, some of which attain very large dimensions. In the Eocene strata of
