Nucleobranchiata.—Those fossil univalves, which in their symmetry resemble the Nautilus, but are unfurnished with air-chambers, have been compared to the recent Heteropoda (or Nucleobranchiata, Bl.), and especially to that division typified by the tiny Atlanta, in which the animal can with draw itself completely into its shell, and close the aperture with an operculum. The genus Porcellia, characteristic of the carboniferous age, has a discoidal shell, with a spiral nucleus projecting, as in Atlanta, from the right side ; the whirls are exposed, and marked with a narrow band along the back, ending in a deep slit (fig. 17, 6). Another genus (Bdlerophon) resembles the recent Oxygyrus in its more globose form, with a similar narrow umbilicus on either side (fig. 17, ; some times the shell is thin and the aperture expanded, like a trumpet, whilst other species are globular and solid ; the for mer may have been tenanted by large animals living at the surface of the open sea, the latter seem to have been more adapted to protect their owners crawling over the bottom, for it can scarcely be insisted that all were necessarily floaters on account of their organization. The species of Bellerophon are numerous in all the palttozoic rocks, and some of the smaller kinds appear to have been gregarious : those with disconnected whirls have been called Cyrtolites (Conrad.) The Bellerophina of d'Orbigny (fig. 18, II), is a minute shell found in the gault. The other division (Firolidee) consists of Mollusks in which the shell is wanting or rudimentary, and small compared with the bulk of the animal. A single species of the genus Cari naria has been found in the middle tertiary of Turin.
Strombid,ce.—The Strombs with their massive shells, never theless, resemble the fragile Heteropods in some respects. They have the same lingual dentition, and the same carnivo rous habits ; and though living on the sea-bed, they rather leap than glide, having a narrow sole and a deeply-divided opercu ligerous lobe. Characteristic of the warmer zones of existing seas, they are only found fossilized in the newer tertiary strata of countries south of Britain; but there is a group of little shells related to the recent Strombus fissurellus in the older tertiaries of London, Paris, and America, to which Agassiz has given the name Rimella. The allied genus of scorpion-shells (Pterocera), now peculiar to eastern seas, has been described as occurring fossil in the secondary strata of Europe ; but the extinct species appear to be more nearly related to Aporrhais. This genus, now confined to the western shores of Europe, occurs in all the tertiaries, and is represented in the secondary rocks by many remarkable forms. Some have been separated under the name Alaria ; and to this group the so-called Pterocera Bentleyi may perhaps be referred (fig. 18, 2). .Rostellaria and
Serapis (or Terebellum), now peculiar to the Red or eastern seas, are conspicuous fossils of the European eocene, at which time their range extended to America. Some of the ancient Rostellarias have the outer lip enormously expanded, as in the R. amply (Hippocrene) of the London clay. In the oolites and chalk there are slender fusiform shells (Spinigera, d'Orb., fig. 18, 2) with spines on the sides of the whirls, as in some recent Roneike.
Muricid.ce.—The great family of whelks, by fax the most important group of living sea-shells, is scarcely of higher anti quity than the eocene tertiary. The Pumpurina of the oolites (fig. 18, 3), and Columbellina of the chalk, are extinct genera, somewhat resembling Purpura and Columbella. But since the so-called " cones " of the oolites have proved to be Tornalellcr, A:Secondary Univalves.
it may not be unreasonable to distrust these other presumed affinities. The huge univalve of the chalk, which Sowerby called a Dolium, has been described as a Pterocera by d'Orbigny. In the tertiaries siphonated univalves abound, and are mostly referable with certainty to recent genera. The only marked change consists in the comparative abundance of some scarce existing forms, and the absence or rarity of many now most conspicuous. Moreover, the geographical distribution of the genera has undergone a great change since the close of the eocene period. This change is most noticeable in the cold temperate zone, and is evidently the result of altered climate.
The northern seas must ever have been inclement, and the tropical seas always tropical ; but the latitude of England being most liable to vicissitudes of climate, might be expected to show the greatest variety, and the most complete and rapid alterations of organic life. In the London clay are found many Tertiary Univalves.
t. Nautilus (Aturia) zic-zac, Shy.; Eocene, Britain.