smooth or slightly striated valves, accuminated at the beaks. The shells of Terebratulæ of every age manifest the beautiful punctation to which Dr Carpenter called attention, and are usually smooth; while, in marked contrast with its associate through the immensity of time, Rhynchonella is deficient of this punctation, and is commonly ridged in radiating folds.

If we place before us a series of Lingulæ according to their antiquity, and include the recent species, we remark the absence of the genus from the British Tertiaries, and the abundance of it in Paleozoic ages, the greater comparative breadth of some of the older species (Lingula Davisii, L. granulata), the more elliptical contour of particular examples (L. elliptica from the Carboniferous rocks, L. Beanii from the Inferior Oolite), but upon the whole a uniformity from one end of the series to the other, which suggests very strongly the idea of very narrow limits imposed on the tendency to variety in this genus of mollusks.

If we take in like manner a series of Terebratulæ, such as T. hastata, from the oldest known forms in Devonian and Carboniferous rocks, T. elongata of the Permian, T. punctata of the Lias, T. ornithocephala of the Inferior Oolite and Fullers' Earth Rocks, and T. digona of the Great Oolite, or another

series such as T. sacculus of the Carboniferous, T. sufflata of the Permian, T. perovalis of the Fullers' Earth, T. intermedia of the Cornbrash, T. biplicata of the Green Sand, T. semiglobosa of the Chalk, T. grandis of the Crag, and T. vitrea of the existing ocean,-illustrate each largely by examples with sinuated or even margins, more or less prominent beaks, greater or smaller foramina, we shall be amazed at the small amount of real differences which divide these species so far-how very far!-removed from one another in order of time. If we try on a mingled series of Terebratulæ of all ages the sharpest powers of our differentiating naturalists, the same result may happen to them as has happened to older palæontologists,—to confound species differing in age from the Devonian Limestones to the Upper Green Sand; from the Chalk to the Bath Oolite; from the Lias to the Mountain Limestone. Little difference appears between T. striata of the Chalk and T. caput serpentis of the Sea, nor is T. fimbria of the Inferior Oolite very unlike the recent T. Australis.

If we choose among Rhynchonellæ a series such as

R. decemplicata, Silurian; R. anisodenta, Devonian; R. pleurodon, Carboniferous; R. variabilis, Lias; R. tetraedra, Marlstone; R. media, Fullers' Earth; R. obsoleta, Great Oolite; R. inconstans, Kimmeridge Clay ;—

or compare

R. Wilsoni of the Silurians; R. concinna of the Oolite; R. octoplicata of the Chalk; with R. psittacea of the Norwegian Seas;

we shall perceive how very small is the amount of change which all the lapse of time has witnessed in the forms of what seem to be among the most variable as well as the most numerous of fossil shells.

Rudista are not known below the Cretaceous

Series.

Monomyaria.-Classing these in four great families, Aviculidæ, Pectinidæ, Limidæ, and Ostreidæ, which are all found both fossil and recent, we remark in the first place the constancy of the general characters of each. Thus Ostreidæ shew always rudely laminated shells; Pectinidæ and Limidæ are nearly equivalvular, neat, and radiated; the former nearly equilateral, the latter more oblique ; but in this respect yielding to the Aviculidæ, which are more frequently smooth externally, and pearly within, and have very unequal valves. No true oysters occur below the Mesozoic strata. Pecten and Lima are not known below the Carboniferous Limestone; Aviculidae belong to every geological age.

Dimyaria.-Two great divisions constitute this large group of shells. To the Asiphonida, which approach nearest to Monomyaria, and include three marine groups, Mytilidæ, Arcadæ, Trigoniadæ, and one

freshwater group, Unionidæ, belong nearly all the equivalved bivalves of the Lower Palæozoic, and the larger portion of those found in the Upper Palæozoic formations. They are numerous in the later strata, and excepting Trigoniadæ still remain so. The larger group of Siphonida begins to be plentiful in the Oolitic and Cretaceous rocks, and is much more abundant in the tertiary strata and existing oceans.

Two conspicuous fossil genera, Trigonia and Pholadomya, are represented by one living species to each. The former is confined in a fossil state to Mesozoic Strata, and in a recent state to the Australian shore; the latter, a constant companion in Mesozoic Strata, is found also in the Eocene beds, and has been discovered living off the Island of Tortosa. The recent species is in each case judged to be distinct from the fossils. What strong affinities, however, obtain in each case between the fossil and the living races will appear by attending to the form, surface ornament, commissure of valves, hinge, umbones, and muscular impressions.

Pholadomya in every age preserves a striking conformity of characters-radiating ribs on a part of the surfaces, usually swollen at intervals by prominent laminae of growth, a thin tumid oblong shell, prominent beaks, gaping posterior end, obscure hinge-teeth. The recent species differs but

slightly from fossils which might be selected from the Oolites.

Trigonia exhibits more diversity of form and ornament, but preserves the essential characters; a thick shell, ribbed or tuberculated with beaks bending toward the posterior side, which is angular, and bears a prominent ligament often preserved in the fossils. Internally the shell is nacreous, with deep muscular impressions; the hinge-teeth are strong, radiating, and transversely striated. If we place in the order of their existence the following species of Trigonia, we shall perceive at once the general affinity and the special diversity which runs through this genus. The recent species is the only one with ridges radiating from the beak over all the surface.

Pteropoda.--These floating mollusks are few in the stratified deposits of every age; more frequent in the sea. Conularia, Theca, and Pterotheca, belong to the Paleozoic Strata; Hyalæa, Cleodora, and Cuvieria, are both recent and tertiary.