broad expansion of the outer lip, and emarginate base, without coloured markings."

It appears, therefore, that the statement upon which M. Deshayes depended amounts to this:-that Mr. Sowerby had seen drawings of shells said to be in the possession of Mr. Hall and Mr.Jennings, resembling in some respects Volùta Lambérti.

The fact mentioned by Mr. Sowerby of the correspondence in colour between the crag volete and the drawings to which he refers is a suspicious circumstance, because the deep ochreous tint exhibited by the specimen figured in the Mineral Conchology is a character more or less common to all the fossils found in the red crag, depending, in all probability, upon the presence of hydrated oxide of iron. It is, however, satisfactory to have learned thus much, that M. Deshayes has not personally examined any recent Volùta Lambérti, and it now only remains for him to name the individuals whom he explicitly states to have really done so, and to learn of them the collection or collections in London in which these rarities are deposited.

I anticipated some difficulties in instituting the present enquiry, which I was led to enter upon from the consideration that a solution of the obscure points connected with the history of this shell would be of importance to those who are interested in our own tertiary deposits; and, perhaps, not less so to those engaged in the study of recent conchology. Should I be fortunate enough to obtain any more facts relating to the subject, I shall not fail to take the earliest opportunity of making them public.

The gigantic species of Terebrátula represented at fig. 13. forms one of the numerous additions to fossil conchology which have resulted from, the examination of those tertiary beds which are interposed between the crag and London clay in some parts of Suffolk. This shell cannot, it is true, be looked upon as an entirely new fossil, since Sowerby has, in the Mineral Conchology, figured and described several specimens, which are undoubtedly young individuals of the same species. The figures now given of this singular fossil are drawn by Mr. James de C. Sowerby, from specimens in my own collection; and, although the most perfect, are by no means the largest which I have seen; having occasionally met with fragments indicating a length of five or six inches, a size considerably exceeding that of any known fossil or recent Terebrátula. Sowerby only remarks of this shell that it is a very abundant crag fossil, and that the valves are never found joined, and always much worn.* In the red crag, whence Sower

Sowerby's Min. Con., vol. vi. p. 148.

a, Terebrátula (natural size) from the coralline crag of Sudburn, Suffolk : b and c, a smaller specimen of the same species, with the valves in contact.

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by's specimens were obtained (by Mrs. Murchison), this is certainly by no means an abundant fossil; the other observation with respect to it is perfectly correct, as I have never met with it there but as a solitary valve. I believe, however, that this shell, in common with many others, known as crag species," has been introduced into that deposit at the expense of an older formation, by the operation of causes analogous to those which are now indiscriminately mingling the Mollusca of the crag with those of the German Ocean. It is only in the beds beneath the crag that this Terebrátula occurs naturally grouped, and, when found there, the valves are most commonly in contact, but we should naturally expect to find them dislocated when washed out of their original matrix, either by the encroachments of the sea, or by the action of a river upon the bed in which they may have been deposited.

Sowerby designated this species by the term variábilis, from observing that many specimens of it were much more orbicular than others; now, this variation in shape, constantly exhibited by immature shells, is shown only in a very slight degree by such as have attained the size of fig. 13. a.

There is one distinction between the young and adult shells deserving particular notice; it consists in the production of the margin of the dorsal valve of the latter. During the early stages of growth, the edges of the valves do not encroach upon one another, there being only a simple adaptation of the margins in an even line, sometimes accidentally distorted, from the excessive thinness of the shells at the line of junction. When, however, the shell has attained the length of 3 in. or more, the front edge is rather suddenly produced,

External view of fig. 13. a, showing the production of the front margin.

with an abrupt termination as at fig. 14. which is received into a notch in the opposite valve. This locking together of the

valves is not, as in most instances, attended with a depression of the produced portion in the one, and a corresponding elevation in the other, interrupting the regularity of the oval form exhibited by the two shells when in contact. The dorsal, or perforated, valve is the more convex, and its apex, or beak, is produced, and in young specimens considerably incurved. The foramen for the passage of the byssus is always circular, and sometimes large enough to admit the tip of the little finger. In adult shells the length, measuring from the perforation to the opposite margin, is about one fourth greater than the width. The lines of growth are very clearly defined, the shell being in other respects perfectly smooth. The interior of the dorsal valve is thickened towards the apex in a most remarkable manner, and to an extent which sometimes diminishes the capacity of a considerable portion of it by one third or even more. The singular appendages attached to the ventral, or lower, valve of this genus, are often preserved unbroken; but, from their extreme delicacy, it is almost impossible to remove the substance which has filled the cavity of the shell, without effecting their destruction.

The Terebrátula is a particularly interesting genus to the geologist, from the vast numbers in which it is universally found throughout nearly all the secondary formations, and in the more ancient fossiliferous strata; while a considerable number of species are still in existence, though these latter bear but a small numerical proportion to the amount known only in the fossil state.

So far as the researches of geologists have at present extended, it would seem that this genus ceased to be abundant at or prior to the commencement of the tertiary epoch. I believe that no instances are on record of the occurrence of any considerable number of species in a deposit belonging to the supra-cretaceous group. The extensive destruction of species which in this instance took place over an immense area, apparently at the same period, will hardly be referred to the agency of any of the causes now in operation by which extermination is thought to be effected; more especially, too, as the living shells are found at various depths, and in the seas of nearly all climates. Terebrátula variábilis is, I think, the only tertiary species yet described as occurring in this country, though three or four are said to exist in the British seas. A small, compressed, longitudinally striated species is abundant in the London clay of the Isle of Sheppy; and, besides the one now figured, Mr. Searles Wood possesses two others from the coralline crag, neither of which is known in the living state.

Another genus of Branchiopodous Mollúsca, Língula, now only found in the seas of hot climates, is associated with the Terebrátula in the coralline crag. No legitimate inferences, respecting the temperature of the period when this formation was deposited, can be drawn from the latter of these two genera, since, as I have already mentioned, it is one capable of supporting various temperatures.

is

The cliffs of Norfolk and Suffolk have suffered an amount of degradation, within the historic period, which would lead us to suppose that some hundred square miles, along this line of coast, may have been swallowed up by the encroachments of the sea, since our eastern tertiary deposits have occupied their present level. Now, when we consider that the crag often little else than a mass of shells, having an average thickness of perhaps 18 ft. or 20 ft., and that some thousand individuals must often be contained in the space of a cubic yard, it is easy to conceive that, could we only have access to the deposits now forming in the neighbouring ocean, a difficulty would sometimes arise, in endeavouring to distinguish between those fossil and recent shells which may there have been embedded during any very long period. In all probability we should meet with new forms, derived from the extensive destruction of tertiary deposits, while we should also expect to find some species of recent Mollusca with which we are as yet unacquainted. In this investigation, an attention to specific distinctions would avail us nothing, and the shells of both periods might have been so acted upon by external agents as to have removed those adventitious characters which each of them formerly possessed. Even now specimens of Túrbo littòreus are sometimes found in the crag of Norfolk which exhibit scarcely any appreciable difference, when placed beside dead shells of their existing analogue. I admit, however, that the instances I have supposed would be exceptions to the general rule, but it is otherwise with the Testacea which we find thus associated in the red crag. Here the organic remains have indiscriminately acquired one common ferruginous aspect, which has superseded all other characters; and it is utterly out of our power to determine whether the animal belonging to these worn and solitary valves of the Terebrátula existed during the formation of both deposits, or only during that of the coralliferous beds.

Here, perhaps, it will not be irrelevant to notice the means by which the age of the upper crag has been determined. 111 species taken from it, collected at different times and by different individuals, upon being examined by M. Deshayes, were found to include 66 extinct forms; and the crag was