bottom not rounded, but defined by two parallel lines. The posterior half of the guard is slightly compressed laterally, and a very faint trace of a longitudinal impression may be discovered on each side, rather nearer the ventral than the dorsal surface. One of the specimens of the Belemnites Owenii, presented to the College of Surgeons by Mr. BRODERIP, measures eleven inches from the apex of the guard to the basal partition of the phragmocone: the diameter or breadth of that partition is one inch and a half: the length of the guard from its apex to the beginning of the crushed alveolus is six inches: the length of the ventral groove is one inch and a half.

With respect to the guard, I have little to add to the excellent descriptions of former authors, besides an account of its microscopic structure*: but this is the more requisite, as in some recent and estimable works the spathose structure is still regarded as the result of accidental posthumous infiltration. The guard consists of numerous, thin, for the most part concentric, layers of minute prismatic trihedral fibres, placed at right angles, or nearly so, to the planes of the layers the crystalline fibres are indicated by lines which radiate from the central axis and cross the lines of growth the lines which define the fibres, when magnified 150 diameters, are seen in many parts of the section to run in pairs with a minutely and gently undulating course, resembling the tubes of dentine, but differing in the transparency of the intercepted calcareous matter, which is like that in the wider spaces separating the pairs of lines.

These differences in the intervals of the radiating fibres may depend on the different parts of the prismatic fibres divided in preparing the sections made parallel to their

course.

There is an appearance not uncommon in microscopic sections of the spathose guard, which, though due to minute splintering or abrasion of the surface, is too characteristic of the texture of the guard to be passed over; it is produced by a number of elongated triangular specks §, defined by their opacity when the section is viewed by transmitted light, and by their white or silvery surface when viewed by

* This was first noticed by Dr. CARPENTER in his valuable paper On the Microscopical Structure of Shells,' in the following words :-"The solid conical sheath of the Belemnite is a very favourable subject for microscopic examination; and of this I have made numerous sections. The greater part of these appear to have almost completely lost any indications of organic structure they may have once presented, being almost or completely homogeneous in their aspect. In some, however, I have met with a structure, which so closely resembles that of the massive Septaria gigantea, that I have no hesitation in the belief, that the shell of the latter is the nearest living analogue of the sheath of the Belemnite. I consider the term 'fibro-calcareous,' applied to the latter by Dr. BUCKLAND, to be therefore erroneous; the structure being referable to the general type of membranous shells. The membrane seems to have been corrugated in a radiating direction, and as the deposition of calcareous matter followed the same, an appearance of radiating fibrous structure is given on fracture, which exactly corresponds with that of Septaria. The only difference between the two cases seems to have been, that, in the Belemnite the deposit took place from within outwards, whilst in the Septaria it was from without inwards.”—Extract from a Paper on the Microscopical Structure of Shells, by WILLIAm B. Carpenter, M.D. Read to the Royal Society, December 22, 1842.

† Pl. VII. fig. 1, B.

Ib. fig. 1, y; fig. 2, d.

§ Ib. fig. 1, ɛ.

reflected light: the long axis of these triangles is always parallel to the fibres, and transverse to the layers; but on one side of the section the apices are turned towards the centre, on the other side towards the circumference of the guard; so that a slight change of focus reverses the position of the triangular specks.

The lines produced by the concentric layers of growth are seldom marked with equal distinctness; the strongest ones are usually in groups of three, five, or eight, with fainter lines in the clearer interspaces: this appears due to the varying thickness of the animal membrane at the contiguous surfaces of the layers of the prismatic fibres, which membrane was either formed by, or afforded attachment to the extremities of the delicate membranous cells which served as moulds for the calcareous matter of the fibres. The impress of these extremities has produced on many of the lines of growth a minutely undulating or crenate course. The layers of growth vary slightly in thickness; many are brought into view by applying a magnifying power of 150 diameters to a transparent section of the guard which otherwise would escape notice; those only being visible to the naked eye that are separated by the thicker lines; they are thus seen to be much more numerous than the septa of the phragmocone. In a transverse section through the middle of the alveolus, eighty layers could be counted in a thickness of a line, and more than three hundred in the solid part of a guard whose semidiameter was four lines.

A longitudinal section of the guard through its centre, and a transverse one, both demonstrate the longitudinal course of the radiating fibres, the linear indications of which might be interpreted as the folds of a plicated membrane: a longitudinal section taken near the circumference of the guard cuts across the radiating fibres, the extremities of which are thus seen in transverse section; their distinct and independent character and trihedral form are then clearly demonstrated, as in Pl. VII. fig. 2 : they vary a little in size, the average diameter being 200th of an inch.

Through a great part of the thickness of the guard next the outer surface, the thin fibrous conical layers are concentric, progressively increasing in length as they approach that surface, and thus forming the alveolus or cavity for the phragmocone. The innermost or first-formed layers are not parallel with the outer ones, but recede from them at their anterior end, contracting as at the opposite end, but in a less degree, so as to form a slender cylindrical stem, sometimes slightly dilated at the end which is in contact with the apex of the capsule of the phragmocone. The interspaces thus left between the earlier and the later layers of the guard are occupied by more abundant animal membrane, mixed with coarser opake calcareous particles, like those which harden and render brittle the corneo-nacreous capsule of the chambers; a filamentary tract of the same kind of matter is continued from the apex of the above capsule down the centre of the guard to its posterior apex.

Mr. PRATT has obtained from the Oxford clay at Christian-Malford the guard of a young Belemnite, three lines long*, fusiform, slightly contracted posteriorly;

* Pl. II. fig. 4.

this must have belonged to an animal just excluded from the egg, if excluded at all. The layers of growth immediately added to this axis extend beyond it, both anteriorly and posteriorly; and, being thicker in the middle than at the two extremities, render the guard fusiform, as in the young specimen rather more than an inch in length* from the same stratum and locality as the preceding. At this stage of growth, which lasts longer in some species, as the Belemnites subfusiformis, than in others, the spathose guard has been mistaken for the spine of an Echinus, and this idea, first entertained of Belemnites in general by KLEIN, has been reproduced in later times by M. RASPAIL: even the sagacious Mr. MILLER, who has done so much towards the elucidation of the Belemnites, was led to conceive the fusiform guard of immature individuals to be remains of a distinct genus, for which he proposed the name of Actinocamax.

The exterior surface of the spathose guard of the Belemnites of the Oxford clay, though smoother than in some other species, is minutely granular, and occasionally presents faint traces of vascular impressions, proving it to have been invested by an organized membrane of the living Cephalopod. On two specimens from this conservative stratum, I have detected remains of a more immediate investment of a thin friable layer of white calcareous matter, analogous to that of the outer layer of the sheath of the phragmocone. The animal membrane or constituent of the spathose guard has been alluded to by Dr. BUCKLAND as being evidenced by the odour resembling burnt horn, produced on burning this part of the Belemnite. I will only add in reference to the spathose calcareous constituent, that its microscopic structure proves it to be an original formation, deposited in membranous cellular moulds, under the influence of the vital organizing forces, and not to be the result of post-mortem infiltration of mineral substance, into an originally light and porous, or cellular texture, as WALCH, PARKINSON, LAMARCK and DE BLAINVILLE § have conjectured.

As respects the phragmocone and its investing sheath, the well-preserved Belemnites of the Oxford clay demonstrate that the sheath is continued backwards to line the alveolar cavity of the spathose guard, as well as forwards from its basal outlet to form the visceral chamber anterior to the phragmocone. The phragmocone in these specimens appears broader than it actually was on account of the compression

* Pl. II. fig. 5.

+ His conjectural figure of the recent Belemnite (Geological Transactions, new Series, vol. ii. (1826) Pl. IX. fig. 15) is essentially the same with those which have been since published by Dr. BUCKLAND, M. D'ORBIGNY and M. DUVAL-Jouve.

M. DUVAL (Mém. sur les Belemnites, 4to, 1841, p. 68) has demonstrated, what MM. DE BLAINVILLE and D'ORBIGNY suspected to be, the true nature of the Actinocamax of MILLER.

§ Sur les Belemnites, 4to, 1827, pp. 32, 33. Specimens of the spathose guard have been discovered which have been fractured during the life-time of the Belemnite, and healed; the broken portions having been held together by the surrounding organized integuments, and reunited by the deposition of new layers of the fibrous structure peculiar to the guard. DUVAL-JOUVE, Sur les Belemnites, p. 37, Pl. X.

which its frail walls have been unable to resist; its basal part is usually squeezed flat, but sometimes one of the septa has slipped forwards so as to present its surfaces to the planes of pressure, and so retain its original form: one of these, which shows likewise its natural slight concavity, demonstrates the marginal position of the siphon as in other Belemnites, Pl. II. figs. 3 and 7, .

The septa are composed chiefly of nacre with a thinner layer of white friable calcareous matter on both surfaces, which is seldom preserved. I have found twenty septa in an extent of two inches from the base of the phragmocone; about an equal number of septa dividing chambers progressively diminishing in depth, and more rapidly in width, are indicated, by detached phragmocones, to have extended to the apex of the socket of the guard. The capsule of the phragmocone consists of a thin layer of mixed albuminous and opake calcareous matter, lined with nacre, but with a yellowish smooth outer surface. In Lord NORTHAMPTON's well-preserved specimen, it terminates by a well-defined margin a little in advance of the ink-bag, forming there the true peristome of the shell, as shown in fig. 9.

The entire phragmocone, with its capsule, of these Belemnites from the Oxford clay, has been found not unfrequently isolated and detached, having slipt out of the alveolar cavity of the guard; such specimens are squeezed flatter than those which have remained in and been protected by the guard. The yielding texture of the phragmoconic capsule has commonly caused it to fall into longitudinal folds when compressed, after having become detached from the alveolus. In two specimens in the collection of Mr. CUNNINGTON of Devizes these folds are well-marked, regular, and on the same side of the capsule, forming two longitudinal grooves* in one and three in the other, the intermediate risings being so well defined as to appear like a structure original and natural to the capsule; but their position is different, and the degree of the indentation varies, and in other specimens the partial longitudinal impressions are barely discernible. If the change of form caused by the compression were not borne in mind, these piles of concave plates would seem not to have been adapted to the alveolus, and might thus be made occasions for the revival of the exploded idea of DE MONTFORT, that the phragmocone of the Belemnite belonged to an animal generically distinct from that which had constructed the spathose guard.

Belemnites whose remains have been left in strata percolated by streams containing mineral matter in solution, have the chambers of the phragmocone filled with crystals consequent upon such calcareous or siliceous infiltration: in the present Belemnites from the Oxford clay, no such infiltration has taken place subsequent to interment; we find only the delicate original calcareous framework of the phragmocone, crushed and squeezed out of its original shape, whilst the solid part of the spathose guard, by virtue of its proper primitive fibrous structure, has retained its cylindrical figure. This fact, independently of the microscopic evidence of organiza* Pl. II. fig. 6, n.

tion, suffices to prove that the weight and solidity of the guard had received no augmentation by inorganic influences after death*.

That the shell of the Belemnite was due to the formative forces of the mantle of a molluscous animal, was the sound speculation as to its nature, which our countryman JOSHUA PLATT first recorded in the Transactions of the Royal Society for 1764. The presence of a camerated and siphonated structure in this complicated shell, led WALCH and GUETTARD, and after them most subsequent conchologists, to class the Belemnites with the Baculites, Orthoceratites, and other more simple chambered shells. Mr. MILLER, having detected evidence of the internal position of the belemnitic shell, compared it, as DELUC§ on other grounds had done, with the internal shell of the Cuttle-fish, and first ventured on a conjectural restoration of the entire animal. He thought the Belemnite to have been an intermediate form in the Cephalopodal class, uniting the internal multilocular shell of the Spirula with the laminated calcareous plate of the Sepia, to which the belemnitic guard appears to correspond; and, believing that the Nautilus, Ammonite, Sepia and Loligo, had the same organization (only the Dibranchiate type of Cephalopodal structure being then known), Mr. MILLER placed the belemnitic shell in the body of a Calamary (Loligo), assigning to the terminal fins the office of clasping the guard and retaining it in its proper position; which last idea M. DE BLAINVILLE very justly rejected.

The subsequent discovery of two grades of organization in the class of Cephalopods, consequent on the dissection of the Nautilus Pompilius ||, called for a closer investigation of the affinities of the Belemnites, and led to an attempt to establish a more definite approximation of these with the other families of siphoniferous Cephalopods, now ranked under two distinct orders of the class.

The first evidence that bore directly upon the question of the position of the Belemnite in this class, was detected by Dr. BUCKLAND¶ and M. AGASSIZ ** in specimens of Belemnite from the lias at Lyme Regis, in which the fossil ink-bag was preserved in the basal chamber of the phragmocone, or that formed by the anterior prolongation and expansion of its capsule.

The importance of this discovery depends chiefly on the facts, that the secreting gland and reservoir of the inky secretion common to all the naked Cephalopods do not exist in the recent Nautilus Pompilius, and that no trace of them has ever been

* Such was Mr. MILLER'S opinion; but, as it was unsupported by microscopic investigation, or by any facts like that above cited, it was not accepted by Prof. DE BLAINVILLE and Dr. BUCKLAND.

† In KNORR'S ' Recueil de Monumens des Catastrophes que le Globe de la Terre a essuyeés,' &c. fol. 1768. Mémoires sur différentes parties des Sciences et des Arts, t. v. 9e Mémoire, 1783.

§ Mémoires sur les Bélemnites, in Journal de Physique, 1799, 1801, 1802.

|| OWEN, Memoir on the Pearly Nautilus, 4to, 1832.

¶ Philosophical Magazine, N.S. 1829, p. 388. [One of the specimens discovered by Miss MARY ANNING of Lyme, on which Dr. BUCKLAND's observations were made, has been presented, since the reading of the present memoir, by the Earl of ENNISKILLEN to the College of Surgeons.]

** Ibid. 1834.