The yery fine specimen of palatal tooth of Ptychodus polygyrus, which we figure in Plate IX., from the collection of N. T. Wetherell, Esq., of Highgate, temporarily draws our attention to a class of remains of very considerable interest.

We have not the leisure at the present time for going as deeply into the subject as it well deserves, nor as the mass of valuable materials accumulated since the publications of Agassiz in 1843, and Dixon in 1850, require.

There are also other important points than the mere bearings of more detailed information of the characters of species very possibly to be gained by a study of the singular and marked group of cestraciont fishes. First known, in abundance of individuals, in the Carboniferous age—though not at any time numerous in genera,—and presenting various forms, numerically abundant, in the Jurassic and other intermediate formations up to the Chalk, characterized by its many varieties of Ptychodus, but now dwindled down to a solitary representative in the Port Jackson shark, it is one of those very circumscribed groups in which we ought to find more especially and distinctly marked traces of the transmutation of one species into another, if such transmutation did exist in the past ages of our planet. That the group does present important evidence on this point is certain, but whether sufficient or not to come to a practical and definite conclusion, may be as yet doubtful; although, if collectors will turn to the fossil remains of these fishes in earnest, we may rest assured of good work in this direction being done. By a glance at the British Museum specimens, and a careful looking over of the descriptions and figures in the ‘Poissons Fossiles’ and the ‘Geology of Sussex,’ any intelligent observer would at once see what new additions would be useful for supplying the missing links in the historic and stratigraphical series. We add here a list of the species of Ptychodus exhibited in our National Collection.

In the parish of Baldernock, county of Stirling, and in the neighbourhood of “fragments of stone reared by creatures of clay,” Craigmadden Castle, the pedestrian will find three large stones well worthy of his notice. They are situated in the centre of a bog; consequently, if the said pedestrian desires to observe, measure, and minutely examine them, he must, as three friends lately did, take off boots and stockings and wade through the water that surrounds the stones to within two feet; that is to say, if he desires such articles to remain dry. The bog forms part of an extensive natural amphitheatre, situated in the Craigmadden moor; and the stones have received the name “Auld Wives' Lifts,” from the absurd tradition that they were placed, once upon a time, among the heather by three old women, natives of Baldernock, Strathblane, and Campsie.

The Lifts are considered by some antiquaries to be nothing more or less than a cromlech or sepulchral monument; others, however, think they are an altar which the Druids used. The former opinion is maintained by Dr. Daniel Wilson in his ‘Archæology.’ His words are: “It is remarkable as an example of a trilith, or complete cromlech, consisting only of three stones. Two, of nearly equal length, support the huge capstone; a block of basalt measuring fully 18 feet in length, by 11 in breadth, and 7 in depth. A narrow triangular space remains open between the three stones, and through this every stranger is required to pass on first visiting the spot, if, according to the rustic creed, he would escape the calamity of dying childless.

The classification given in the sequel is based on the following premises:—

1st. The entire area of the British Isles has undergone at different times, during the Glacial and Post-Glacial periods, a succession of secular elevating and subsiding movements.

2nd. At the close of the Pliocene period, the relative level of land and sea over the British area was approximately the same as at present.

3rd. The edge of the two-hundred-fathoms submarine plateau, on the east side of the North Atlantic, formed the west coast-line of a continent (now represented by Europe) during the earliest time (epoch) of the Glacial period.

4th. The climate of the British area was frigid in the extreme during the Glacial period, allowing epochs of amelioration.

5th. Rock-surfaces undergo enormous degradation when they are above the sea-level, during the prevalency of glaciation.

Whenever we begin to think about the formation of the universe we get at once into the realms of speculation, and the only value of our thoughts rests in their probability. In everything unknown we must first form an idea—that is, speculate; then, by partial gatherings of facts, or by positive reasoning, we may theorize. Ultimately, by the accumulation of evidence, we may prove that which, in the first place, we only imagined. When first men observed the sun, they regarded the earth as a flat plain, over which the sun passed in his heavenly course, and below which, at eve, he retired to rest. It was not until many ages had elapsed that the world came to be regarded as round, and even then it was long before the sun was considered as a fixed centre of the planetary system revolving round him.

By no nation of ancient times has astronomy been more advanced than the Greeks. Not that the Greeks ever worked out much to a proved result, but they were an imaginative people, and they invented notions. If one theory or speculation was disproved, they invented another; and, hit or miss, they always seemed to have fresh ideas in reserve. In some things astronomical, as in many other things that the world believes in, we may be heretics, and we admit we do not adhere to all the cosmical, physical, geological, and spiritual tenets of the popular faiths. We may not entirely believe in the perfect stability of the universe; we may doubt the eternal endurance of the sun's bright rays; and we may not quite acquiesce in the unchangeable permanence of tne planetary orbits: in short, we do not believe in the permanence of anything whatever in creation. All ever has been change, and changeful all things ever will be. Diversity and change are visible in the first created things of which any relics have been left us.

In the British Museum there are several specimens of “papilionaceous” flattened bodies exhibiting six or more concentric lines radiating from two nuclei on either side of an elevated medial line. The stone in which they are embedded is unquestionably the Stonesfield slate; it contains the characteristic Trigonia angulata, Rhynchonella, Ostrea, and Modiola Sowerbyana, D'Orb. (= plicata, Sowerby). The enigmatical bodies to which I now allude are stained of a deep redferruginous colour, the matrix retaining the grey tint and crystalline texture of the Stonesfield slate. On comparison between these remains and those of the specimens of Geomyda spinosa, from Singapore, in the British Museum, presented by Sir A. Smith, a comparison which was suggested to me by Mr. Davies, whose accurate discrimination first threw light upon the nature of the present evidence, I have been led to consider that the specimens in the Fossil Gallery represent the second, third, and fourth median scutes of a tortoise allied to the recent African species. The fossils and their corresponding impressions from the Stonesfield slate afford, according to my interpretation, evidence of the texture of the horny scutes which were developed outside the bony carapace of the old Oolitic tortoise. A particular interest is attached to these specimens, as they were considered by the late Edward Forbes as Trigonellites, or opercula of Ammonites.