nature, are evanescent and perishable; defying all artificial preservation of their genuine form, and leaving the inquirer no other object to speculate upon, than an empty, inanimate covering.

It has, nevertheless, been found, in proportion as a more correct knowledge of these beings has been slowly acquired, that a general uniformity of structure in the shells of any particular group, is so frequently accompanied by a corresponding similarity of organization in the animal, that little doubt can remain of this being, with certain limitations, a general rule: and that although we may be totally ignorant of the precise nature of the one, yet that we are perfectly justified, by analogical reasoning, to class and arrange its shelly covering in an artificial system; waiting for that knowledge, which will hereafter give us a more accurate insight into its natural affinities.

The truth of these remarks will appear very obvious, on looking to the genus Helix, as it was left by Linnæus, and as it was considered only a few years back; when the French writers (who have been foremost in the necessary task of forming new divisions) still considered it only in the light of a genus, containing many hundreds of species. The illustrious Lamarck perceived the utter uselessness of such a classification; he seized upon the most prominent types of form, and at once gave them a character and a name. The peculiar views of M. Ferussac led him, in the first instance, to return to the old arrangement, so far as to consider these shells merely as a genus, divided into subgenera, sections, &c. This view, however, he seems at length to have gradually abandoned; and virtually to admit what, indeed, is quite obvious-that they constitute a family, and a very extensive one, comprising numerous minor groups, or genera, many of which rest on striking dissimilarities in their animals, and all on certain and obvious characters in the shell.

The great error which, until lately, methodists have fallen into, has been that of considering no group in the light of a genus, unless its limits, or separation from that which was supposed immediately to follow it, could be clearly defined. This notion, still very prevalent among continental naturalists, has been fast losing ground in this country, since the writings of Macleay have thrown a new light upon the economy of nature,

and struck out a path which is now followed, almost universally, by British naturalists. To characterize a new form, and to give it a name, is no longer looked upon as a dangerous innovation. Slight modifications of structure may, indeed, sometimes be mistaken for types of a superior group, and placed in a station which they may subsequently be found not entitled to. This, with our present confined knowledge, is inevitable; and if it be a real evil, it is still a very insignificant one, when put in comparison with that, which, from a system of generalizing, leads to the neglect of minute discriminations and rigorous comparison.

Without a knowledge of the animal inhabiting these shells, it is impossible to say anything on its natural affinities. Yet, so far as we can judge from the shells themselves, they appear intermediate between Lamarck's Bulimus and Achatina: between A. fasciata and achatinella pica, there is, indeed, a much closer resemblance than at first appears; yet they clearly belong to two distinct geographic groups.

ACHATINELLA.

Testa ovato-conica, spiralis.

Columellæ basis truncata, incrassata. Labium

internum nullum, externum internè incrassatum, margine acuto.

Habitat in Oceani Pacifici Insulis.

Shell oblong-conic, spiral. Columella with the base thickened and truncate.
Inner lip none, outer lip internally thickened, the margin acute.

Inhabits the Islands of the Pacific Ocean.

(Generic Type. Monodonta semi-nigra Lam.)?

The shells forming the present group are all of a small size; the largest not equalling an inch in length. In general appearance they resemble Bulimi, both as regards form, and the proportionate length of the spire, the principal whorl being more or less ventricose; but in some it is sufficiently short to render the shell trochiform. This circumstance, joined to the thickened and somewhat projecting base of the columella, induces me to believe, that the proposed type of the genus has been mistaken by Lamarck for a marine shell, and described, in his Systeme, under the name of monadonta seminigra. This supposition cannot, however, be verified, unless by a reference to the specimen he described; it is also rendered somewhat doubtful, as he does not quote the figures, in Dixon's Voyage round the

World, which (although I have not the book at this moment before me) accurately represent my A. pica. In this, as well as in all the other species, the thick and abruptly truncated base of the pillar gives it the appearance of an obtuse tooth, covered with a white enamel. The extreme margin of the outer lip is acute, but it is internally bordered by a thickened rim. These characters, in all the species I have yet seen, are strongly developed, and render this group one of the most conspicuous in the family of Helecina.

1.—A. pica.

A. testa trochiformi, nigrá; apice columellæque basi albis.
Shell trochiform, black; apex and base of the pillar white.
Monodonta, semi-nigra Lam.?

Shellths of an inch long, body whorl convex, spire conic; the three upper whorls white or fulvous, without any convexity, and forming a conic point. Suture thickened, and margined by a sulcated groove: a character that runs through all the following species, except A. acuta. Interior of the aperture, and base of the pillar white; the latter tinged with rose colour: margin of the outer lip within, bordered with black.

2.-A. perversa.

A. testa sinistrorsa, sub trochiformi, fusca faciis transversis nigricantibus lineisque longitudinalibus; apice suturâque albis.

Shell reversed, sub-trochiform, fulvous brown, with darker transverse bands, and longitudinal lines; apex and suture white.

Shell less trochiform, but somewhat larger than the last. The terminal whorls of the spire are likewise formed in the same manner; these, together with the suture, the pillar, and the aperture, are pure white. The rest of the shell is a drab coloured brown, variegated by transverse blackish bands and lines; and sometimes by others, in a waved direction, near the suture. The spiral line, which follows the suture, and the tip of the shell, both of a pure white, renders this a very elegant species.

3.-A. acuta.

A. testa ovato-oblonga, castanea, fascia marginali fulvâ; spirâ longiusculâ, apice acuto, nigro.

Shell ovate-oblong, chestnut, with a marginal fulvous band; spire somewhat lengthened, acute, the tip black.

Shell somewhat pyriform; the spire being pointed, and con

siderably longer than the aperture. The colour is a deep reddish chestnut, the suture having a marginal band of fulvous white, but without any groove. The apex is blackish; the pillar twisted, and but slightly thickened.

4.-A. livida.

A. testa sinistrorsâ, ovata, obtusâ, lividè-fuscâ; spirâ incrassatâ, sutura fulvá.

Shell reversed, ovate, obtuse, livid brown, spire thickened, suture fulvous.

A small, unbanded species, scarcely exceeding half an inch long, and perfectly resembling, in form, the green variety of bulimus citrinus. The three specimens, now before me, are reversed; varying from a light olive brown, to a livid purplish colour, which lies in longitudinal shades, and gradually changes to white on the terminal whorls of the spire; the suture is marked by a narrow band of deep fulvous: aperture white, tinged with purple.

5.-A. bulimoïdes.

A. testá ovato-oblongâ, subventricosâ, albente, fasciis castaneis ; spirâ incras satá, apice fusco.

Shell ovate oblong, sub-ventricose, whitish, with chestnut bands; spire thickened, the lip pale brown.

Larger than the last, and nearly of the same form; but the spire is less thickened, and more pointed at the apex. The ground colour, in some specimens, is pale chestnut or ferruginous, banded with darker shades, and another of pure white: in others the upper half of each whorl is whitish, and the lower chestnut, marked by darker bands: the suture is scarcely, if at all, margined by a groove; the aperture and pillar white.

Var.?-(rosea.)

A. testá sinistrorsâ, pallidè roseâ, fasciis albis obsoletis.

Shell reversed, pale rose-colour, with obsolete white bands.

I place this, for the present, as a variety of the last, to which, except in being reversed, it bears a close resemblance in size, form, and general habit. It is entirely of a pale and delicate rose colour, with two obsolete bands of white on the body whorl; the margin of the lip and columella are of a deeper rose colour, and the aperture white. It should be observed, that the marginal groove, which is scarcely perceptible in the last, is, in this, very distinct.

6.-A. pulcherrima.

A. testá ovato oblongá, sub-cylindraceâ, albâ vel flavá, fasciis castaneis ornatis ; labii margine fusco.

Var. a. aurantia, suturá castanea.

Shell ovate oblong, sub-cylindrical, white or yellow, with broad bands of chestnut; margin of the lip, brown.

Var. a. golden yellow, suture chestnut.

This very elegant species is about eight-tenths of an inch long, and is much more slender than any of the preceding. It varies somewhat in form, some specimens being more ventricose than others, and also in the number and colour of its bands. The ground colour is a deep and rich chestnut, with from one to three bands of orange, yellow, fulvous, or white: the marginal groove to the suture is very close and distinct in all. The golden yellow variety is without bands: in all the colours are remarkably rich and vivid.

On the Comparative Population of the World, in Ancient and Modern Times.

(Read before the Philosophical and Literary Society of Liverpool.) [Communicated by Mr. MERRITT.]

AFTER the masterly essay of Hume on this subject, in which the balance of evidence is so nicely adjusted, and the ultimate decision so carefully and hesitatingly pronounced, it will appear, to most persons, that the inquiry is quite exhausted. Any one who now ventures to express his dissent from the conclusions of so acute a scholar, and so accurate a reasoner, will subject himself to the danger of being heard with feelings of contemptuous distrust, and will not be expected to prove much, except his own ignorance and presumption. But on a point of this nature, where an approximation to truth is all that can be expected, no authority, however grave, must ever be permitted to arrest inquiry. Hume himself has afforded us a laudable example of what may be called ratiocinative courage, in the question now before us. Some of the highest authorities of modern times not only differ from him on this important point, but differ in so great a degree, as almost to fix a presumption of absurdity on one party or the other.

The errors of Mr. Hume are never on the side of careless