Conida.—The Cones and Pleurotomas appear first in the chalk, and are abundant in the eocene, accompanied by an intermediate form (Conorbis, fig. 19, 3), and another extinct sub genus (Borsonia, fig. 19, 4), in which the column is plaited, as in Mitra. The genus Terebra is more common in the miocene.
Volutida.—The Volutes also appear as cretaceous fossils in Europe and Southern India ; they are very abundant in the London clay, and one occurs in the English crag. The ancient species are mostly distinguished by their spires being acute, as in Mitra (fig. 19, 5), a peculiarity only found in one very rare living (?) species, dredged from a bed of dead shells in 132 fathoms water (792 feet) off the Cape. The crag Volute resembles the Magellanic form. Cymba olla, the only living European Volute, is a fossil in the pliocene of Majorca.
Cyprceidce.—The Cowries form another group of subtro pical shells once common in the temperate zone. Several
large species are found in the London clay, most nearly related to the southern Cyprovula ; whilst the crag contains only members of the sub-genus Trivia, one of which still lives on our coast.
The round-mouthed shells (Holostomata), whether animal feeders or vegetarians, make a conspicuous figure amongst the fossils of an earlier period than that in which the last group began to flourish. The carnivorous Naticide and Pyramide2 licks are represented in the palteozoic strata by Naticopsis, Lozonema (fig.17, z), and Masrochilus (fig. 17, z). The violet snail (Ianthina), so unlike any other existing shell-fish, seems related to the Silurian Scalites, Raphistoma, and Holopea. Shells like Scalaria and Solarium occur in the bias and oolites associated with Chemnitzice (1) of extraordinary size, and species of Eulima and Niso. These families of shells and the Geri thiaike are more abundant fossil than recent, the known num bers being 1500 extinct and 900 living forms. Solaria, with disconnected whirls and pyramidal opercula (Bifrontia, Dh.), are common in the eocene tertiary, and a single living species (B. zanckea) has been discovered by M'Andrew.
Amongst the tertiary Naticas are many with an oblique aperture and peculiar perforation (G/obidus, J. Sby., = Ampul lina, and others with prominences on the pillar (Deshaye sia, fig. 19, 6.) The Nerinceas of the oolites are remarkable for the spiral ridges (like the " worm" of a screw) winding round their interior, and giving rise to the variety of singular pat terns seen in sections (fig. 18, 4). A similar structure exists in the recent " telescope-shell" (Terebralia). The fresh-water univalves of the Wealden and older tertiaries differ but little from their recent congeners of the genera Paludina, Potamides, Melania, and Melanopsis. Fossil Turritelke are of doubtful occurrence before the tertiary ; the Silurian species have the peristome complete (Holopella, 111‘C.) ; another form (Proto, fig. 19, 7) is characteristic of the miocene.