ters not, I conceive, at what angle or in what direction the said cone of rays proceeding from the large metal be thrown by the diagonal one, provided these conditions are fulfilled, (supposing of course the position of the small metal to be the centre of the tube, so that it shall truly receive the whole of the light of the great one.) We may, therefore, either adjust the small metal to the eye-tube or the eye-tube to the small metal, or we may do both, which latter will probably be the most expedient, and is the method I have preferred; I have effected it in a very simple manner, by having the tube made to fit loosely into another wider piece, which is screwed on in the usual way, to the side of the telescope:-the vacancy between them is filled up with wax, the inner tube is tight at the bottom of the external one, by the interposition of a small setting chamferred at the edge, but admits of a slight rotatory motion towards the eye-glass by heating the wax with the flame of a candle which is inserted into the external tube, and which unites them both together; time will be given to adjust it before the wax cools, when it will all set tight, and will not be liable to get out of order. Two small niches should be made, one in the shoulder of the screw of the external tube, and the other in that of the female screw to which it is applied, to be a guide that the two pieces may always be screwed home to a particular point: or it is very probable the adjustment may be spoiled, because it will be a chance if the eyetube when fixed, is precisely at right angles to the side of the telescope. A variety of methods of effecting this adjustment will present themselves to the workman, instead of that which I have used, which though it answers very well, yet does not look very elegant or scientific. Thus, instead of the wax, three screws might be used, fixed into the external tube; or such a contrivance as is represented in Fig. II., by having counter screws to play against those by which the setting for the eye-piece is attached to the rackwork, on the side of the telescope, &c. Now it is not my intention to assert that this adjustment is absolutely indispensable, for I have not a doubt but that a superior workman might execute a long eyetube, such as I have employed, so that nothing but the usual adjustments would be required; still I think that no Newtonian would be

injured by having such an apparatus to it as I have recommended, even though the eye-tube were only of the common length; it is certain it could do no harm at least. I think I can, moreover, with confidence assert that increasing the distance between the small metal and the eye-glass, for the purpose of applying stops, will not be found to make the least sensible difference in the performance of an instrument, as far as the figure of the small metal is concerned, provided it is of the standard goodness; if but imperfect such an alteration will evidently try it more, and this will be shewn by examining a double star which will probably vary slightly in the distance at which the stars appear separated, (cæteris paribus,) according as the eye-glass approximates to, or recedes from, an imperfect diagonal. To conclude, as an Herschelian telescope is nothing but a Newtonian, used without the interposition of a small metal reflector, whatever has been said of the latter, will equally apply to it, and the same principle in the eye-tube and adjustment, will for the same reasons be equally adapted to both, though the manner of execution will be different; I have, however, made no experiments on this kind of telescope.

[The portion of this paper relating to Microscopes is reserved for our next Number.]

ART. IV. The Characters of several New Shells, belonging to the Linnæan Volutæ, with a few Observations on the present State of Conchology. By William Swainson, Esq., F.R. and L.S.

THE study of conchology has now become so general, or, if I may be allowed the term, so fashionable, that the number of elementary works is truly surprising. The new systems of the French conchologists have been translated, explained, and advocated, in various publications; while the admirers of the Linnæan method,

*It is evident that if the diagonal metal were quite perfect, it could make no difference at what distance the eye-glass was placed from it; if decidedly imperfect, it is no less plain that the nearer the eye-glass is placed to it the better, because the less of the edges will be called into action, which will of course be the worst part.

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have not been backward in expressing their warm attachment to the plan of the great Swedish naturalist. It is not my present intention to speculate upon the respective merits of these systems. In the study of no class of the animal kingdom have there been so few absolute facts discovered, whereon to build a truly natural system, as in that of the testaceous mollusca. In the history of those families which are known, anomalies have been discovered, which baffle explanation, and obstacles almost insurmountable, from the very nature and habitat of the animals, conspire to retard that rigid investigation of their economy, which must alone form the basis of their perfect arrangement.

But while so many writers have been engaged in forming systems and constructing genera, the elucidation of species has comparatively been neglected.

An extensive acquaintance with species is the first step to a knowledge of natural divisions. In every branch of natural history those who have seen and studied the fewest individuals, will be most apt to create new genera; "when they have seen more, they will discover the intermediate links which unite different genera; and thus be forced to join what they formerly separated *”. I am fearful this has not sufficiently been considered by the authors and advocates of the French systems: it may be doubtful if their generic distinctions are not too much refined; but it is certain that a knowledge of the science is daily becoming more unattainable to all but professed naturalists.

While this revolution of classification and of genera is going on, our cabinets are crowded by innumerable species, some of which we know not how to name, while others (well known by the figures of the older writers) remain undescribed: new species are continually pouring in upon us to augment the number: and although the student may be perfect in the elements of his system, he knows not how to proceed, or where to turn, if he ventures on the investigation of species.

The volumes of Lamarck (His. Nat. des Animaux sans› Vertebres) have indeed done much to remedy this evil. They contain • Wildenow, Principles of Botany, P. 175, Sect. 163.

a considerable increase of new species, and a more perfect elucidation of many of the old ones; but, on the other hand, the same over-refinement which marks the characters of his genera, will be traced in the discrimination of his species; this is more particularly the case in his account of the genera Conus, Oliva, and Helix. Let me not, however, be misunderstood, as wishing to depreciate the merits of this great man. His general reputation could not be affected either by my praise or my censure. But obliged, as he has been, to employ the sight of another in finishing his latter volumes, it would perhaps have been better for his own sake, and that of the science, to which he has devoted his long life and great abilities, if they had never been published.

The importance of monographs, or complete histories of particular tribes, or families, in every branch of natural history is unquestionably very great; for their object is, not only to ascertain the limits of genera, and the affinities and analogies, which the individuals of such genera bear to others, but likewise to include the history of all the species thereunto belonging. To accomplish this, however, is in the power of a very few. Access must be had to the rich contents of foreign museums, and of costly libraries, to supply what may be deficient in minor collections; and it is from this cause that nearly all the monographs of extensive families have proceeded from naturalists in the charge of public museums, or in the possession of immense private collections. From the labours of these men science has received the greatest assistance. But, although few can enjoy the advantages such materials afford, considerable benefits will be derived from the labours of those, who frame a correct diagnosis of individual species; particularly when relative characters are subjoined, and comparisons made between others to which they bear a resemblance. When it is considered how many rare and unknown shells have lain for years in the cabinets of mere collectors, and how much greater is the number of those species more usually seen, but which are likewise unrecorded, the value of these isolated descriptions will be rightly understood. They are the indispensable materials for completing a general survey of the natural world, and constitute the ultimate object of

all systems; namely, such a knowledge of the individuals, as will enable the student to assign to each "a local habitation and a name."

I shall now proceed to describe several beautiful shells; mostly of uncommon rarity, and apparently unknown to modern writers: the four first belong to the genus Voluta, as it is now restricted; and the remainder to Mitra, a genus to which I have long paid much attention, with the ultimate hope of illustrating it by a distinct monograph.

VOLUTA. Lam. (Div. 1. Musicales.)

Voluta chrysostoma.

V. testâ ovatâ, albente, lineis angulatis maculisque castaneis ornata; anfractibus spinis brevibus, concavis coronatis; apice crasso, obtuso, lævi; aperturâ aureâ.

Shell ovate, whitish, with angulated chestnut lines and spots; whorls crowned by short concave spines; apex thick, obtuse, smooth; aperture golden.

Voluta chrysostoma. Sec. Exotic Conch. Fas. 5. ined.

Voluta luteostoma? testa obovata, angulata, lineis et venis fuscentibus in fundo albido undulata sub-perforata, anfractibus cinctis nodis conicis, apice obtuso, basi valde emarginata, columella plicata plicis quatuor solidis, fauce lutea. Chemnitz xi. p. 18. tab. 177. F. 1707-8.

DESCRIPTION.

The shell in its habit, approaches V. vespertilio: its total length is about two inches, of which the spire occupies not more than half an inch its form is oval, and its surface without sculpture: the basal volution, and the two first whorls of the spire, are crowned by a row of short thin vaulted spines, rather acute, and resembling those on V. diadema, (Ex. Conch. Fas. 1.) The remaining three spiral whorls are perfectly smooth, the middle one being by much the largest, and the whole forming a thick and somewhat obtuse cone. The base is deeply emarginate, and the plaits on the columella, (which are four in number) are very thick. The ground colour of the specimens before me is nearly white, with broad