a

cently instituted in France, under the title Scissurella. An abstract of the paper, announcing the genus, by M. Alcide Dessalines D'Orbigny, in Memoires de la Societé d'Histoire Naturelle de Paris, i. 340, is given by Mr G. B. Sowerby in the Zoological Journal, i. 255.

M. D'Orbigny's character of the genus, as translated by Mr Sowerby, stands thus: “ Shell univalve, free, umbilicated, with a depressed spire, aperture rather rounded, without a canal, its lips without a peristome and disjoined : right lip notched with a deep slit, following the growth of the volutions, obliterated to within a short distance of the margin, and forming a sort of keel upon the back of the shell.” According to M. D'Orbigny, the shell belongs to the family of Trochidæ, while Mr Sowerby is inclined to consider it as related to the Fissurelladæ. The last observer supposes that the slit in maturity is obliterated at the margin, leaving the peristome entire ; but that immediately behind the margin, there is a perforation left by a portion of the slit not being filled up, as in the genus

Haliotis. Though this last circumstance may take place in some species, I may observe, that in the one in my posses

Ι sion, that part of the body whorl on each side the slit is apt to break off, and thus efface the most obvious mark of

the genus.

Scissurella crispala.--Shell white, and without any apparent cuticle.

The largest specimen in my possession does not equal the fifteenth of an inch. Whorls three, increasing rather rapidly from the slightly elevated apex, and sloping with a gentle convexity from the separating line to the keel. Under side with a central cavity, from which the whorl extends, a little convex, to the keel. The whorls are marked by numerous fine transverse arcuated ribs, narrower than the intervening spaces, crossed by fine longitudinal striæ

a

a

VOL. VI.

B b

(most conspicuous in the spaces), giving to the shell its peculiar reticulated appearance. The ribs on the upper side are coarser than those below. The aperture is suborbicular, slightly depressed; the outer lip thin, the inner lip slightly reflected over the cavity, spread on the body whorl, and continuous with the outer lip. From the pillar cavity a shallow gutter extends anteally, and shortly joins the pillar lip. It is chiefly conspicuous on the larger specimens. The longitudinal slit, in the middle of the outer lip, extends backwards about two-thirds of the diameter of the shell, when it joins the narrow groove in the keel of the shell which had formerly been occupied by the slit. The margins of the slit are slightly elevated, as well as the groove, which is seen winding round at the separating line nearly to the apex.

The
groove

itself is slightly ribbed across.

It seems probable, that the slit is situate immediately over the aërating organs of the animal. All the specimens, when found, were destitute of the animal, thus preventing any observations on its forms; but, from the number of shells collected in one spot, it would appear to be gregarious in its habits.

The preceding description offers one statement which would seem to forbid the insertion of this shell in the genus S. issurella of M. D'Orbigny,--the continuous peristome. In the generic character, the lips are represented as disjoined. Whether this may lead to the institution of a new genus, when a greater number of recent species shall have been detected, or when the fossil species, of which Mr Sowerby, we believe, possesses several examples, shall have been described, we do not venture to determine.

M. D'Orbigny describes two recent species. The first, Scissurella lævigata, is smooth, or marked only by the lines of growth. The second, Scissurella costata, is rib

1

bed, with the lower ones the most prominent and acute, and the pillar cavity surrounded by circular striæ.

The figures annexed (Plate VI. Fig. 3.) will convey a more correct idea of the shell than any description.

MANSE OF FLISK,

26th April 1827.

nb 2

XVII.-Account of an Optical Illusion called the

Fairy Islands, seen on the North Coast of Ireland.

In a Letter from Mr Samuel Thomas Gregg to

Mr STEWART.

(Read 15th December 1827.)

Sir, Having been an eye-witness, with five others, of the following optical phenomena, commonly called the Fairy Islands, in that part of the country where they are seen by the peasantry, but which I take to be the effects of the mirage, I thought an account of them might be interesting to your scientific friends, and I now send the particulars, that they may be added to the facts already accumulated on this subject.

The appearances in question occurred in September, and the morning of the day on which they were seen was so extremely hazy, that the Londonderry steam-packet could not proceed on her voyage till 12 o'clock noon, when the mist cleared off, and the day turned out fine, with a gentle breeze. The wind now changed to the east, and we were informed by the people that these phenomena are only seen when it blows from that quarter. A party, consisting of my father, brother, three sailors, and myself, went out on a boating excursion, from the White Rocks near Portrush, and on our return, at 5 o'clock in the afternoon, we first saw this optical deception make its appearance close to the Skerry Islands, which are situated in the sea, about four miles distant from the main land. We now saw a second set of Islands, at an extraordinary elevation, apparently 200 yards above the level of the sea, breaking off by degrees in the centre, the top branching into appearances resembling old castles, towers, spires, with men and cattle running over them! It sometimes also presented an appearance, which I can only liken (though it may appear whimsical) to a regiment of dragoons galloping and manœuvring! On our first observing the phenomena, the sailors cried out, “ There's the Fairy Islands," -the name given to these optical illusions by the peasantry. The appearances continued for the space of an hour and a half, and we remained a considerable time on our oars observing them,—but the evening began to close in with a thick mist, and the objects vanished from our sight.

The Skerry Islands where these phenomena occurred, are a small group of rocks a few miles out in the and nearly opposite Portrush, being about six miles distant from the Giant's Causeway, Downhill, and Dunluce Castle. I mention these particulars with a view to explain the successive appearances. This phenomenon seems to be caused by the irregular refractions which the rays of light occasionally suffer, by passing through the different strata of the lower atmosphere. Now, there are old castles and spires on the coast opposite the Skerry Islands, where these optical deceptions occurred, and to account for the appearances just mentioned, the rays of light in passing through the lower strata of the air, should describe curves, whose final tangents point in the visual directions of the

a

sea,