Gasteropoda

The bonnet-limpets (Calyptrceidce) are common in the old rocks, which also contain a few species of Chiton and shells like Dentalium. Fossil Trochicke are very numerous, but hitherto many Litorini&e have doubtless been included with them. Perhaps no true Turbo is known from strata before the cretaceous. The Euomphali (fig. 17, 4), which characterize the older rocks, have multispiral calcareous opercula, like the recent Cydostrema (= Ad,eorbis). The genus Madurea (fig. 17, 9), which has been regarded as a " left-handed" Kuompha /us, is probably very different ; it has a thick shelly operculum, sinistrally spiral, and furnished with an internal process, as the Nerites are ; the spire is sunk and concealed, whilst the whirls are exposed on the flattened under-side ; it occurs in the older Silurian rocks of Scotland and North America. One common feature of the pakeozoic spiral shells is their ten dency to become irregular towards the conclusion of their growth : in Serpularia (= Phanerotinus, Sby.), the whirls are all disunited ; in Seoliostonta (fig. 17, 3) and Catantostoma the aperture is expanded. Some small oolitic shells have a thick ened peristome (Crossostoma, fig. 18, spike the recent Lietia, which commences in the older tertiary. A large proportion of the trochiform fossil shells have their whirls, whether round or angular, marked by a peculiar band, terminating in a deep slit at the aperture ; most of these were solid nacreous shells belonging to the genus Pleurotomaria, of which but a single species survives ; others in their slenderness resemble Turri tellee, and have been named Murehisonia (fig. 17, s). The car boniferous shell called Polytremaria has a row of holes in place of a slit ; and the Silurian Tubina (fig. 17, s) has three rows of tubular spines. The Cirrus of the inferior oolite is a reversed shell with one row of similar ornaments ; and Tro chotorna (fig. 18, 6) has a perforation near the margin of the aperture, which is carried onward as the shell grows. Scissur ella, which is always diminutive and not pearly, makes its first appearance only in the newer tertiary. Haliotis occurs in the miocene of Malta. The ilreritidec appear in the oolites ; besides true Nerites, there are Neritomtv (fig. 18, 7), with a channeled outer lip ; Pileolus, which is perfectly limpet-like above (fig. 18, 8) ; and Neritainis, with its angular columellar notch most distinctly marked. Key-hole limpets (Fissurelliclee) occur as early as the carboniferous period, but are very scarce at first, and never become numerous. The oolitic Rimula is a minute shell supposed to be related to a very rare living species. Ordinary limpets (Patellicks) of unequivocal form are found in the Bath oolite, but are afterwards less plentiful, and almost disappear from the tertiaries ; M. d'Orbigny regarded

them as generically distinct, but employed for them a name (Heldon, Mont) synonymous with Patella.

Pidmonifera.

The existence of air-breathing snails in the palreozoic rocks is shown by a small "chrysalis-shell, with a round, not toothed, aperture, (Dendropupa), discovered by Dr. Dawson and Sir C. Lyell in a hollow coal tree of Nova Scotia." Upwards of 40 species of Pupa have been found fossil in eocene strata. The Purbeck limestone contains a modem-looking Physa ; and other species of extraordinary size are found in the older tertiary of France, and also in Central India, where the genus does not exist at the present day. The fresh-water eocene of the Isle of Wight and Paris has afforded many spe cies of Limncea and Planorbis ; a Glandina rivalling in size the G. truncates of South Carolina ; a Cyclostoma, with a sculp tured operculum like the Cyclotus Jamaicensis ; and an elon gated species of the section Megalomastoma, which is now liv ing in both East and West Indies. At Hordle has been found the little Helix labyrinthieus, still living in Texas ; and in the south of France occur representatives of the Brazilian genera Megaspira and Anastoma (fig. 19, 9). In the miocene is found another genus (Ferussina, fig. 19,10) resembling the lamp-snail, but supposed to be operculated. The Pulmonifera of the Eng lish pliocene are in a few instances extinct, at least in England; nearly all are still living here, but more or less abundant now than they were in the times of the mastodon and elephant. The extinct land-snails of the Atlantic islands Madeira and Porto Santo are associated with remains of many recent species occurring in numbers which have relatively altered, telling the same tale of gradual changes, affecting some species prejudici ally, but favourable to the increase of others. The fossil land snails of St. Helena were supposed by Mr. Darwin to have become finally extinct only in the last century, owing to the destruction of the native woods by the instrumentality of goats and swine.

Tectibranchiata.—The families typified by Tornatella,Ringi cula, and Rolla played a more important part in the secondary and tertiary periods, but their affinities have been seldom un derstood. The cone-like Acteonina appeared in the carbonifer ous rocks, and attained a remarkable development in the lias (fig. 18, so). They were succeeded by the Acteonelke, with a plaited columella, in the cretaceous strata ; and by Volvaria (fig. 19, i) in the eocene. The diminutive Ringiculce of our seas were preceded by large species of the same genus in the tertiaries, and by Cinulia (fig. 18, 9), Globiconeha, and Tylo stoma, in the cretaceous strata. The genus Varigera has varices recurring twice in each whirl, like Etdima ; and Pterodonta is winged like Strom,bus.