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the non-continuance of the numerous ridges down the sides. Hybodus formosus differs in the larger ridges, with wider intervals, and a more regular disposition over the sides of the spine. In both H. reticulatus and H. formosus, the halves of the hinder border meet at a less open angle, and the median dentated ridge is more prominent; consequently the hind border meets the lateral surface at a more open angle than in H. complanatus.

From the known species of Acrodus, the present specimen differs in the pattern of the ridging of the sides: the ridges in Acrodus are fine, but regularly longitudinal, and in some specimens almost obsolete. The sides of the enamelled body of the spine are less flat, more convex, in Acrodus, and the spine is thicker in proportion to the breadth in Acrodus than in Hybodus.

The length of the present spine of H. complanatus, is 1 foot 4 inches; the breadth is 1 inch 9 lines; the thickness posteriorly is nearly 1 inch.

The proportions of the enamelled and unenamelled surfaces of the spine, the obliquity of the terminal basal line of the ganoin, and the excavation of the smooth implanted basal part or root, accord with the usual Hybodont type of Ichthyodorulite.

II. ON THE SUPPOSED OCCURRENCE OF PHOLAS BURROWS IN THE UPPER PARTS OF THE GREAT AND LITTLE ORMESHEADS.

By T. G. BONNEY, M.A., F.G.S.

(PLATE XVII.)

N a paper entitled "On Traces of Glacial Action near Llandudno,"

cribed certain indications of glacial action (as they appeared to me) upon the two masses of Carboniferous Limestone, well known to visitors at Llandudno under the name of the Great and Little Ormesheads. The principal conclusion at which I arrived was that, “after the limestone hills of the district had acquired their leading forms by upheaval and marine denudation, the whole district was depressed. The summits of the low rocky islets thus formed became capped with ice-fields, which in places descended in glaciers into the sea. At times, very probably, they were united to the mainland by pack or coast ice." I then supposed that, after some minor changes, "the whole was gradually upheaved above the sea, probably-at any rate, in the case of the Great Ormeshead-not quite uniformly."

This conclusion was opposed by Mr. R. D. Darbishire in a paper read before the Literary and Philosophical Society of Manchester, and published in their Memoirs (ser. 3, vol. iv.), a copy of which he very kindly sent to me. In this, after mentioning sundry indications of marine action, he describes certain burrows which he has discovered on various parts of the above hills, and considers to have been formed by a species of Pholas, probably P. crispata. He comes, therefore, to the following conclusion: "In the preservation of these burrows, in loose beach stones, and in the edges of the tables of out

cropping strata, and in the level sea-stripped scars, I submit that we have final and irrefragable proof that the surface of these anciently submarine hills had not been touched by the iceberg or glacier for some time before, nor at any time since, they respectively emerged from the waves."

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I have recently had an opportunity of examining the neighbourhood of Llandudno, with the aid of Mr. Darbishire's very interesting and valuable paper, and will endeavour to state why on this point I see no reason for altering my conclusion. If I rightly understand him, we are agreed that the district shows signs of both marine denudation and glacial action; the debate, therefore, is narrowed to this one point, whether or not these hills have been capped by ice since they were covered by the sea. Mr. Darbishire argues for the negative answer from the presence of (1) sea-beach and beach marks; (2), Pholas burrows.

(1). Among these he enumerates certain large blocks lying on the upper plateau of the Great Ormeshead, "the remains of the play or rage of the waves about the head of the new-born island, and as such emphatically a raised beach of the grandest kind," and several nearly horizontal surfaces of limestone in the same neighbourhood, "cut and furrowed, and worn in fissures, potholes and other forms, very similar to those of the like beds in like positions in the intertidal spaces below." These blocks I had described, figuring one of the largest (Plate XII. Vol. IV. p. 289) as most probably blocs perchées. That this is a gigantic stranded boulder is placed beyond dispute by a very brief examination of the locality. Its base is flat, and rests upon three slight projections in the rough shelving scar of limestone, upon which it lies. The only question is by what agency it has been conveyed to its present position. This may have been done :

(a). By the force of the waves, as suggested by Mr. Darbishire. No doubt the position of this, and the numerous other similarly situated blocks, might be accounted for in this way; but in that case, I think, we might fairly expect to see a more beach-like arrangement of the boulders, a larger quantity of rounded pebbles, and (as the elevation of the Head can hardly be more recent than the deposition of the lowland 'till' with its numerous boulders of trap and foreign rocks-often ice-scratched) a considerable sprinkling of boulders from the neighbourhood of Conway. Mr. Darbishire himself acknowledges that he has "not found anywhere at the higher level one block of any other rock than Mountain Limestone.” The weathering of the limestone scars in the neighbourhood, which he brings forward, with some reserve, as a proof of marine denudation, does not differ from what may be seen on most horizontal surfaces of limestone, and may be produced by the action of the atmosphere and of freshwater impregnated with carbonic acid gas. I am familiar with similar markings in many limestone districts far away from the sea, among others in parts of the higher Alps.'

1 During my examination of the large boulder mentioned above, I was led, in a search for Pholas' burrows, to look narrowly into a crack about an inch and a

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(b). By being dropped from floating or stranded bergs. objection to this is the difficulty of understanding, without assuming a considerable alteration of level, whence these blocks could have been derived, and the evidence (as it appears to me) in the contour of parts of the Great, and more especially of the Little Ormeshead, of glacial subsequent to marine action.

(c). By being moved to their present position by land ice. The difficulty in this explanation is that the only broken crest of rock now visible (to the N.W.), from which such a boulder is likely to have been derived, is but little higher than it. This crest, however, is higher than the base of the boulder, and the rock to the S. is still more elevated, although it is now a bare scar. Possibly some of these blocks may represent a former extension of the 'reef' over this now denuded portion. There is also much higher ground to the S.E. Mr. Darbishire appears to feel a difficulty in regarding a block as 'perched,' unless it has travelled from a distance, in saying, "They are uniformly of a stone apparently identical with the beds in their immediate neighbourhood, and may often be connected with a neighbouring reef of rock, both by the character of the stone, and the style of wearing. A few may be found actually in situ, as isolated pillars or tables," etc. (p. 14.) I never supposed that they had travelled more than a few hundred yards, at the most; and the distance a block is conveyed by a glacier, will be little, or great, according to the size and form of the ice-stream; nor did I intend to imply that I considered the Ormeshead glaciers to be large. All the indications of an extensive glacier system are wanting. Therefore, notwithstanding some difficulties, I still adhere to my opinion that many of these scattered masses of limestone rock are true blocs perchées.

(2). If, however, Mr. Darbishire is right in attributing the peculiar pits and depressions in the limestone rock to the excavating action of some species of Pholas, the question is pretty well settled; for although under certain circumstances the sea (as may be observed on many parts of the west coast of Norway) has surprisingly little effect in obliterating the traces of glacial action, it is improbable, to say the least, that any would have survived in a rock so liable to denudation as carboniferous limestone, and in a position so exposed as the Ormesheads. But are these pits Pholas burrows? quarter wide, indicated in the middle of the side towards the spectator, in the plate. At a depth of about three inches it appeared to be choked with angular fragments of limestone, among which could be seen a shell, which, when uncovered a little, proved to be Buccinum undatum, wedged in with its spire downwards. As it had a very ancient appearance, I supposed that this would make for the marine theory; but a little poking at it with a chisel showed that the fragment in contact with it above was bone, probably of a sheep. Beneath the Buccinum, was a shell of Helix aspersa. Further examination of the crack showed that it extended to the upper part of the stone, and widened in that direction, so as to be at last three or four inches across. It was filled with limestone fragments to within a foot or so of the top; on removing the topmost of these, I found a fresh bone (part of the clavicle (?) of a lamb). I suspect, therefore, that the Buccinum, with the bones, were dropped down the crack by a raven or a gull. I was confirmed in this view by finding that I could not succeed in extracting them from the crack by the lateral aperture.

I have long had suspicions that they are not; and the very fact of their discovery some 1,400 feet above the sea on the hills of Derbyshire, appeared, by proving too much, to do worse than prove nothing. Is it not in the highest degree improbable that such burrows, so easily weathered, should have survived the atmospheric waste of so many centuries? What is there to show that these hills of central England have been submerged since the period when the valleys of Wales and the Lake-districts were filled with glaciers; and, without very strong corroborative evidence, are we justified in assuming such an inequality of upheaval as would be required in this case? If, however, the Pholas burrows belong to an earlier epoch, say to that of the drift shells on Moel Tryfaen, is it likely that they would have escaped the action of ice and weather during the subsequent glacial period? But to return to the supposed Pholas burrows at Llandudno. Most of the limestone of which the neighbouring hills are composed appears to have a tendency to weather in generally oval or circular pits; so much so, that not unfrequently a face of rock appears to have suffered from a kind of gigantic small-pox. Still, besides these and other cavities, often curiously regular, which may be set down to atmospheric action in one form or another, there are a considerable number occurring at various elevations above the sea-roughly speaking, almost all over the Great Ormeshead-which cannot be thus accounted for. Of these I select two groups for especial description.

The first is in a block of limestone which projects from the steep turfy slope on the northern face of the Great Ormeshead, a few yards below the pathway which surrounds the mountain. It is in the line of a slight glen or depression by which a narrow track leads to the upper plateaux, some three or four hundred yards to the east of St. Tuduo's Church. The burrows are clustered about a natural subangular step on the north-western face of the block, the lowest being about a foot from the turf. The annexed sketch (Plate XVII., Fig. 1) will show their general character better than any description. The following are some notes on the burrows numbered in the figure:-(1). About an inch in width, and two in depth, it curves gently upwards, having a small aperture in the side at the point." (2). Does not extend any further into the stone. (3). Extends, curving gently upwards, for about 1 inches; this contained a small specimen of a banded Helix. (4). Extends nearly four inches upwards into the stone, presenting in section through its axis, roughly, the form indicated in the accompanying figure. (5). Descends slightly. (6). About one inch deep, and at right angles to the channel in which it is placed. (7). A curved channel, about one inch wide, not extending up into the stone. (8). A curved channel, rather irregular in section, perhaps a natural depression. (9). A curved channel, 1 See note and references at the end of Mr. Darbishire's paper.

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