Trias to Recent. Some of the earlier forms described as Natica belong to Naticopsis.

Littorina (the Periwinkle).-Shell thick, almost globular, with few whorls, the last being large. Mouth rounded, more acute posteriorly. Outer lip sharp at the edge; inner lip flattened at the columella. Surface commonly marked with growth-lines and spiral striæ. Outer layer composed of calcite (Sorby). The animal inhabits the shore, sometimes forming considerable shellbanks.

Lias to Recent.

Nerita. Shell thick. Spire very small; last whorl very large, and prolonged out rather more obliquely than in Natica, so that the total effect is not so globular. Mouth semicircular; outer lip commonly thickened and set with little ridges directed inwards; inner lip generally also ridged. Columella flattened. Surface smooth, or spirally ribbed.

Lias to Recent. Mostly Cainozoic. The Mesozoic forms have the typical shape, but most of them have smooth lips, the outer lip being, moreover, sharp in many examples.

Neritina.-Close ally of Nerita, the shape being similar. Outer lip sharp, not thickened; inner lip marked by ridges. Surface ornamented with coloured lines and spots, which are preserved even in many fossil specimens. Typically fresh-water; sometimes brackish.

The form of the shell and its mouth-characters will not serve to distinguish Neritina from some of the early Neritas, which are, however, undoubtedly marine.

Eocene to Recent. Perhaps Mesozoic.

Naticopsis.-Form like Natica, but expanding very rapidly. No umbilicus. Mouth approaching oval Operculum convex outwardly, and not spiral.

Devonian to Trias.

Turbo.-Form typically turbinate, sometimes approaching Littorina. Mouth round, lips not meeting to form a continuous border, part of the mouth being bounded merely by the surface of the whorl. Surface of shell often spirally ribbed. Operculum thickened with calcareous deposit till it becomes outwardly almost hemispherical, the massive examples from large living species being sometimes used as ornaments. See Trochus. Ordovician to Recent.

Trochus.-Allied to Turbo, but broadly conical, and somewhat flattened below. Mouth without continuous lips, but more angular than in Turbo. Operculum horny only. The genus has been much divided into sub-genera. Some fossil forms, the

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opercula being absent, may possibly be referred to Turbo; but the difference of outer form is characteristic.

The living species are mostly shallow-water forms.

Gotlandian to Recent.

Phasianella-Ally of Trochus. Shell like a rather elongated Paludina; often fairly large. Mouth oval; outer lip thin. Surface smooth.

Cretaceous to Recent. Earlier forms have now been referred to Bourguetia.

Euomphalus. Shell fairly large. Almost discoidal, the spire being very low. Mouth more or less polygonal, with a slight notch not far from the suture. Umbilicus extremely wide. The earlier part of the shell is sometimes seen in sections to be cut off by partitions.

Gotlandian to Carboniferous.

Pleurotomaria.-Shell like Trochus, but sometimes with a low spire. Mouth fairly round; outer lip with a deep slit in it, which runs back along the whorl. As growth proceeds, this slit is closed over, and leaves a band-like mark or a ridge running spirally round the shell about the middle of the whorl. Umbilicus sometimes present. Surface of shell generally handsomely ornamented with spiral ribs and knobs; but no trace of these appears in casts, which are common in some formations and which are not identifiable with certainty.

Cambrian to Recent. Abundant in earlier Mesozoic; very rare at present day.

Murchisonia.-Shell turreted; slit in outer lip, as in Pleurotomaria, and similarly closed by a band as the shell grows. Surface smooth, or with longitudinal ridges.

Cambrian to Trias; especially Devonian and Carboniferous. Bellerophon.-Formerly often classed as a Heteropod. Shell shaped like a rather open Nautilus, the spired character typical of the gastropods being wanting. Mouth widely expanded and fairly circular, the shell coiling over inwards symmetrically in the centre of its inner lip. Outer lip deeply notched, a corresponding band running round the exterior of the shell.

The shells are often distorted and compressed in the older formations, and the expanded character of the mouth becomes partly lost. Distinguish carefully from Cephalopods. Cambrian to Permian.

Galerus (Calyptræa in part).-Shell rather like that of the

Limpet (Patella), being flatly conical; but generally with a trace of spiral winding at the apex. Below, a spirally bent plate, which is free anteriorly and is attached along its posterior margin, crosses what would otherwise be a wide unbroken mouth. The under side of the apex is thus partly partitioned off. In Calyptræa proper this plate is represented by a spoon-shaped or bent process dependent from below the apex.

Galerus is Upper Cretaceous to Recent.

Patella.-The Limpet. Shell like a flattish cone; mouth oval; apex of shell nearly central abeve it. Surface usually radially ribbed.

Cretaceous to Recent.

Note.-Shells allied to Patella occur in the oldest Cambrian strata; in the absence of the animal, it is difficult to refer shells of this type to existing genera.

B. PULMONATA.

The animal breathes by a lung-sac in place of branchiæ. The group is almost entirely fresh-water or terrestrial. Delabeche (Geol. Observer, 2nd. ed., p. 122) gives the specific gravity of land-shells as 2.82 to 2.87. The material is thus probably aragonite.

Helix (the common Snail; fig. 103).-Shell variously shaped, but commonly rather flatly conical; at times approaching dis

Fig. 103.-Helix occlusa

(Oligocene).

Fig. 104.-Planorbis euomphalus (Oligocene).

coidal. Mouth obliquely semicircular or oval; outer lip sometimes slightly expanded and thickened; inner lip represented, as in Turbo, partly by the surface of the whorl. Sometimes, however, a calcareous thickening occurs in the position of the inner lip (fig. 103). Rather broad umbilicus. Surface commonly smooth, with mere growth-lines. Terrestrial.

Eocene to Recent. Species very numerous at the present time. The fact that the animal lived on land may account for its apparent absence in earlier periods, sub-aerial deposits being rarely preserved.

Bulimus.-Elongated ovoid, or turreted, often of large size. Mouth oval, not oblique, and rather long from anterior to posterior end. Thickened and continuous lips, which are sometimes expanded. No umbilicus. Surface commonly smooth, or only slightly ornamented. Terrestrial. See note on Helix.

Upper Cretaceous to Recent.

Limnæa-Shell particularly thin and fragile. Elongated oval, with large final whorl. Mouth large, rounded anteriorly, elongated from front to back. Lips thin and sharp. Columella twisted obliquely. Surface smooth. Fresh-water.

Purbeck to Recent. Especially Cainozoic.

Planorbis (fig. 104).-Shell delicate, as in Limnæa; various in form, but typically discoidal, the spire being very short, and the coils almost in one plane, as in Euomphalus. Mouth semicircular to oval. Outer lip sharp; inner lip represented by the surface of the whorl. Umbilicus very broad. Surface commonly smooth, with mere growth-lines. Fresh-water.

Jurassic to Recent. Especially Cainozoic.

X. Pteropoda.

While the small and commonly conical calcareous shells of several of the Thecosomata, or shell-bearing Pteropods, are found in Cainozoic deposits, some large Paleozoic genera occur, which differ widely from the modern type. Their reference to the Pteropods must still be considered provisional, since, in the face of such strange forms as Calceola among the Corals and the Hippuritidæ among the Lamellibranchs, these conical and sometimes operculate shells may belong to groups, the other members of which differ from them in external aspect. Mr. G. F. Matthew has, it may be noted, referred Hyolithes to the worms (Trans. R. Soc. Canada, vol. vii., 1901, sect. 4, p. 101).

Hyolithes (Theca).-Shell small or large, even reaching 20 cm.

in length. Triangular in cross-section; straight, tapering, forming a long steep pyramid. Aperture at the wider end of the An operculum shell, its border lying obliquely to the axis.

has been observed fitting into it. Commonly marked with oblique striæ. These shells are often crushed flat in early Paleozoic shales, but the long pyramidal form remains recognisable. L. Cambrian to Permian. Almost all are early Palæozoic. Conularia. Shaped like a straight steep-sided pyramid, as in Theca, and similarly variable in size. Some species are slightly curved at the posterior end. Four-sided, giving square, or approximately square, cross-sections. Each side bears a shallow longitudinal furrow. Commonly marked with oblique striæ.

Septa have been observed, dividing off the lower part of the cavity.

Cambrian to Lias. Almost all are early Paleozoic.

Tentaculites.-Often described as the tube of an annelid. Surface Rather small, steeply conical; circular in cross-section. marked with ring-like ridges, which lie in planes perpendicular to the axis of the shell.

Ordovician to Devonian.

XI. Cephalopoda.

The shell-bearing forms of these highly developed molluscs. were in former ages far more numerous than at the present day; and the species are often sufficiently constant and widely spread to serve in marking special palæontological zones. At the present time we have a prevalence of one great dibranchiate order (which includes the ordinary cuttle-fish, Argonauta, &c.), while the tetrabranchiate type (Nautilus) is decadent.

We must leave for zoologists the many interesting questions that have been raised in the last few years, and content ourselves with placing the common fossil remains discussed under the broad headings Nautiloidea, Ammonoidea, and Phragmophora, without question as to the dibranchiate or tetrabranchiate character of the animals that gave rise to them.

The shells are sometimes straight, sometimes coiled over at one or both ends, sometimes closely and spirally coiled throughout; but, with very few exceptions, the coiling, when it occurs, takes place in one plane, so that the distinction between these shells and the ordinary gastropod univalves is marked. The regular partitions (septa), by which the cephalopod shell is