On the Collection of British Fossil Cephalopoda.—In the Tertiary formations of England, the remains of but seven species of Nautilus (comprising Aturia) have been noticed; the large species (N. imperialis) is the most common. These are generally in a good state of preservation, and only require the careful removal of the surrounding clay or marl. When pyrites largely enters into the composition of the specimens, the investing matrix can seldom be effectually cleared off: if the outer surface, and general form, be not well displayed, breaking the specimen will often expose the inner cells, with the siphunculus, in a beautiful state. The Nautilus imperialis is occasionally imbedded in the septaria of the Isle of Sheppey, and of Bognor and Bracklesham, on the Sussex coast. Sections of such examples, in the vertical direction of the enclosed shell, afford, when polished, very brilliant and interesting fossils; the septa and the shelly tube of the siphunculus are often preserved.
The Cephalopods of the Cretaceous formation, with the exception of those in the argillaceous strata of the Galt, are generally destitute of their shells, and only occur in the state of casts; and the Chalk Nautili are liable to separate at the divisions of the septa, and an entire series of the casts of the chambers may sometimes be obtained, so as to display the entire form of the original shell. The Ammonites of the White Chalk, although mere casts, yet retain their configuration, the foliated margins of the septa dove-tailing them together. I have already mentioned that search should be made along the back of these specimens for the siphuncle, the shelly tube of which is sometimes well defined. In the Chalk-marl the casts are sharper than in the White Chalk, and generally of a deep ochreous colour, with the lines of the sinuous septa clearly defined. The siphuncle is occasionally preserved in pyrites, in the Ammonites, Nautili, Turrilites, and Scaphites; and the outer lip or margin of the mouth, or aperture, of the latter, and of the Ammonites, is frequently replaced by the same mineral.
The Ammonites, Hamites, &c. of the Galt have their pearly coat remaining, but this investment is extremely delicate; and although when first removed from the marl it is beautifully iridescent, the vivid hues are very evanescent, and the shell becomes opaque and of a light fawn colour. Very commonly the shell flakes off, wholly or in part, leaving a cast of indurated pyritous marl. I have preserved specimens with the shell many years, by applying a thin coat of mastic varnish with a soft camel-hair pencil, before the marl had become dry, and while the shells were entire. The Galt Ammonites, like the Nautili of the London Clay, are often invested with pyrites, and have the inner cells and siphuncle well preserved.
The argillaceous strata of the Oolite and Lias contain Ammonites, &c. in much the same state of mineralization as those of the Galt. The Kimmeridge Clay, in some localities, particularly around Aylesbury (and especially at Hartwell Park, the seat of Dr. Lee), abounds in Ammonites with the shell as perfect and beautiful as if just dredged up from the sea. But, like the fossils of the Galt, few of the specimens are durable; although in many examples the shell may be preserved by the application of mastic varnish. The most common Ammonite at Hartwell is A. biplex (Sow. Min. Conch.), which varies from three inches to one foot in diameter; the surface is covered by very strong ribs that encircle the whorls. The shell is thick, and composed of several laminæ.[429]
[429] According to the observations of my son, the outer layers, when highly magnified, present an appearance of opaque areolæ, with irregular radiating fibres; the inner laminæ are covered with minute pores, apparently the orifices of tubuli, some of which are arranged singly in crescents, and others are confluent, like short strands of beads. I mention the fact to direct attention to the microscopic examination of the structure of these splendid fossils.
The sparry casts of the separate cells of Ammonites which occur in some of the calcareous beds of the Oolite, will not fail to be observed by the collector. It is convenient to preserve such specimens either on a tray or board, in which a groove is made for their reception, or in a mould of gutta percha.
In collecting Belemnitidæ, the caution already given, of examining the surrounding clay or marl, must not be disregarded; the student should remember, that traces of the soft parts of the animals, even of mere impressions of the body and head, with the tentacula and their acetabula, or little horny rings and hooks, are more important than the most splendid examples of the spathose durable osselet. The guards should be selected with especial reference to their containing the phragmocone (see [Lign. 141], fig. 2), or chambered conical shell, in the alveolus or cavity of the upper and larger end. An apparently worthless fragment of a Belemnite will often be found to possess this part of the structure, as in the example figured, which, until fractured longitudinally, had been thrown by among useless duplicates. The search for the remains of the fossil naked Cephalopoda, as the Teuthidæ and Sepiadæ, and their ink-bags, must be made in a like cautious manner. In the Lias marls, the ink-bag and its duct is often found partially covered by a pellicle of nacre, without any trace of the other parts of the animal. A reference to Dr. Buckland’s plates (Bd. pl. xxviii. xxix) will familiarize the student with the appearance of these fossil remains.
LOCALITIES OF FOSSIL CEPHALOPODA.
A FEW BRITISH LOCALITIES OF FOSSIL CEPHALOPODA.