Lias bed that supports it, and seven hundred feet above the level of the sea. Fig. 1, will give you an exact idea of this deposition, and those of which we have been speaking in our former conversations, as they have been traced in this part of the country.

ANNE. I find my ideas on the subject much cleared by these little sketches. It is difficult without them to get rid of the idea of horizontal beds, regularly covering each other.

MRS. L.-I need not tell you, that of these Oolitic formations are the Freestones, so valuable for building in those parts of the country where they abound, at Bath especially. The minerals found in this lower series are few. Calcareous Spar is of course abundant, as it always is where there is much Lime. The organic remains of vertebral animals are very rarely found in the inferior Oolite. Fragments of the claws of Marine Crustacea, such as Crabs and Lobsters, occur at Dundry and in Oxfordshire. Of the Testaceous remains, there are many varieties of Nautilus, Belemnites, &c., which have been mentioned before. Here is a specimen of Terebratula, I think you have not seen, Fig. 2-do you perceive what description of shell it is?

ANNE.-I suppose an Inequivalved Bivalve, because one shell is larger, and wrapping over the other.

MRS. L.-True. Of the Echinus family there are many species-I think you have not seen one. It contains many Genera, called Cidaris, Clypeus, &c.; but they come under the general name of Echinus, and have all a similar character, which you will recognize by examining Fig. 3. It is of the Genus of Echinus, called Ananchytes, and is not known but in the fossil state, though many of the family are found recent: you have doubtless seen them covered with spines, projecting from the tubercles of the shell.

MAT. They are inhabitants of the sea, I believe.

MRS. L.-They are so as well as most, if not all the Fossils of the Inferior Oolite. But to return to the

Freestone beds, the most important of these Strata, generally distinguished by the name of the Great Oolite. They vary in thickness, from one hundred and thirty to two hundred feet, and in value according to the different degrees of hardness. The Freestone of Northamptonshire is rendered very beautiful by the distinctness of the ovate particles: that of Bath has generally a finer grain. The repairs of Westminster Abbey are of this stone. St. Paul's is built from the Oxfordshire quarries. Fragments of shells may be discerned throughout the stone, but so broken down, it is generally impossible to distinguish their species; hence we have little knowledge of the fossils that belong to it. In the uncovered hills about Bath, the structure of these rocks may be distinct. ly seen. "The Great Oolite forms a flat table land on their summits, ending with an abrupt edge; this is succeeded by a gentle slope which marks the Fuller's Earth, and a degree of verdure is observed, arising from the springs that flow over these clay beds: beneath is the lower terrace of Inferior Oolite, which breaks down, steep and precipitous, to the valley. In many instances, large masses of the Great Oolite, having broken from its escarpment, are spread over the slope of the Fuller's Earth, presenting a scene of ruin resembling the cliffs of the Isle of Wight."

Fig. 4, is a Tronchus buried in Inferior Oolite.

SERIES OF FAMILIAR CONVERSATIONS ON THE ANIMAL KINGDOM.

CONVERSATION XVIII.

CLASS-MOLLUSCA.

The Muscle, Pinna, Snail, Cowry, Sepia, and
Nautilus.

HENRY.-I find that Linnæus classed Oysters, with

all other animals that have shells, under the general name of testaceous worms, or shell-fish; and that his order of Mollusca consists only of those simple animals which are without shells, and are furnished with tentacula

or arms.

PAPA. He did; but as I have already remarked to you, it was for want of a more correct acquaintance with their structure and properties. They certainly bear no resemblance to fishes, except that most of them are inhabitants of the sea; and therefore-as accuracy in terms is of great importance to the accuracy of our ideas-I object against calling them by that name.

HENRY. The power which the Muscle has of spinning threads is a singular property.

PAPA. They do not spin them, but cast them in a deep groove or furrow which runs along the foot, and serves the purpose of a mould. This they accomplish by seizing with the point of the foot the gluten which is supplied by a gland situated at its base, and drawing it along the groove. When the thread thus formed is sufficiently hardened, they conduct it by the foot to the place they intend to fix themselves to, and fasten it there; and when it is fastened, it is said, they swing themselves round to see if it be sufficiently strong, and then proceed to cast another. In this manner a Muscle will cast four or five threads in a day. These threads, which are commonly called its beard, enable it, like a ship at anchor, to brave all the agitations of the water. The Pinna, or Ascidia tribe, are the most remarkable for the production of these threads-they form them sometimes a foot and a half long, and of such a beautiful and silky texture, that the inhabitants of Palermo manufacture them into gloves and stockings; which are, however, too warm for common wear. This animal is called in Italy and Provence, The Silk-worm of the Sea. ANNA. Is it found in England?

PAPA.-Yes; a bed of one species of them, the Seawing, was discovered a few years ago in Salcombe Bay,

in Devonshire. They inhabit a gravelly bottom, covered with mud and long sea-weeds, on which they may be seen standing erect, firmly fixed by their strong threads with their broad end about an inch above the surface. ANNA. How large are they?

PAPA. They vary very much in size; and may be found from an inch to a foot in length.

HENRY.-I think I have read, that the Byssus of the ancients was a production of a species of Pinna.

PAPA. So Aristotle informs us; but the name seems to have been used indiscriminately by other writers for any spun material that was finer and more valuable than wool.

ANNA.-You said, Papa, the other day, that snails belong to the Mollusca class. I have brought one in on purpose to hear something about them.

PAPA. I shall readily tell you all I know. The helex or snail is more highly organized than any of the acephalæ it has, you know, a head; a mouth furnished with a tongue and teeth, with several other organs nearly as complete as they are in the largest animal.

ANNA.-Its eyes, I think I have heard, are in its

horns.

PAPA. Some naturalists have thought so from the care with which the animal draws them in when touched. Indeed, they have even undertaken to describe the form of their eyes, and to say that the vitreous, the aqueous, and the crystalline humour might be found in them. But others are of opinion that snails and slugs can neither see nor hear, and that their horns are merely very fine organs of sensation: which opinion is correct, I am unable to decide.

HENRY. It is really very discouraging, Father, to one who is just entering the paths of science, to find so many uncertainties lie in his way.

PAPA. And he will find these uncertainties multiply, as he proceeds. You must remember, however, that to know that we cannot know certain things, is in