met with in connection with any of the simple or typical forms of fossil chambered shells, as the Orthoceratites, Baculites, Ammonites, &c.

Reflecting on this marked difference in the anatomy of the Nautilus, I was led to perceive that, as this rare Cephalopod derived sufficient protection from its large and strong external shell, it might well dispense with that peculiar glandular apparatus which had before been deemed a common character of the Cephalopodal class; whilst on the other hand, as the highly organized naked Cephalopods enjoy active powers of locomotion which would be incompatible with the incumbrance of a heavy external testaceous defensive covering, they required a compensatory endowment of the power of secreting an inky fluid, which, when alarmed, they might inject into the surrounding water and conceal themselves by the dusky cloud thus occasioned. But the branchial character of the naked order of Cephalopods is an essential condition of their muscular powers; the presence of an ink-bladder, therefore, in the extinct Belemnites, would have implied the internal position of the shell, even if other proof had been wanting; and, by the laws of correlation, it implies likewise the presence of the muscular forces for rapid swimming, and the concomitant conditions of the respiratory, the vascular and the nervous systems. Connecting, therefore, all these considerations with the detection of the ink-bag in the shell of the Belemnite, I felt justified in referring the Belemnites, and likewise the Spirula, on account of the ascertained internal position of its shell, to the Dibranchiate order; and I therefore separated these chambered and siphoniferous shells from the Nautilus and the Ammonites, in the classification of the Cephalopoda submitted to the Zoological Society in February 1836*.

But the true grounds of this separation seem not to have been appreciated or understood by some Palæontologists. Professor PHILLIPS, for example, in his excellent article on Turrilites, in the part of the Penny Cyclopædia published in January 1843, has observed, "The relations of Turrilites, Scaphites, Baculites and Hamites to Am

* Zoological Transactions, ii. pp. 127, 129. "The Cephalopods with internal chambered shells, heretofore classed with the siphoniferous Cephalopods, which constitute the preceding order, I would join with all the other naked Cephalopods, to form a second order, under the term Dibranchiata, having reference to the number of gills, viz. two. This number is constant in all the Seiches' of CUVIER, and is associated with the presence of two branchial hearts, besides the single systemic heart, and with an ink-bag: there can be little doubt that the same type of structure is exemplified in the Spirula, from what has been determined respecting its external characters." "The discovery by Dr. BUCKLAND of the remains of the ink-bag in the extinct Belemnites, justifies the conclusion from the laws of coexistence, that these Cephalopods also possessed two gills and two branchial hearts." In the same year, 1836, the Number of the Cyclopædia of Anatomy and Physiology was published, containing my Article Cephalopoda,' in which it is argued of the Belemnites :-" As it is certain that the animals of this family of extinct Cephalopods possessed the ink-bag, they must consequently have been enveloped by a muscular mantle; and we may, therefore, infer that they resembled the Dibranchiates in their locomotive and respiratory organs, and consequently in the general plan of their organization. In the structure and position of their siphoniferous camerated shell, they are intermediate to Spirula and Sepia, and as the animal of Spirula is proved to be a Decapod, the probability is very strong that the animal of the Belemnite was of the same type."-p. 520,

monites is very obvious; and, as through Goniatites, this great extinct group is certainly connected to the living and extinct Nautili, Mr. OWEN has ventured to include them all in the Tetrabranchiate Cephalopoda, leaving Spirula and the Belemnites with Sepia and the Dibranchiate types. However this may be, the determination of the relative affinities among the numerous fossil Cephalopods, a point of great importance, must be worked out with the help of other considerations than the respiratory system." In the memoir on the 'Classification of Cephalopoda,' above cited, it will be found that many other considerations than those of the respiratory system, and of equal importance with them, influenced me in forming an opinion of the natural position of the Belemnites in the class Cephalopoda.

LEOPOLD VON BUCH, who believed that he could trace in certain slabs containing Belemnites the impressions of the Cephalopods to which they belonged, concluded "that the body of the animal enveloped the greater part of the shell, and exceeded its length by eight or ten times*." Other considerations taken from the shell itself prove, as has already been shown, that it was wholly internal.

The specimen presented to the Hunterian Museum by the Marquess of NORTHAMPTON, exhibits the phragmocone, the muscular mantle, a small part of the head, and a greater or less proportion of six of the cephalic tentacula which are armed with horny hooks in a double alternate series, as in the Onychoteuthis gigas.

The phragmocone with the soft parts of the Belemnite has been detached from the guard probably soon after death; and the whole squeezed nearly flat after becoming interred in the laminated clayey matrix. The resistance of the ink-bag (n) with its inspissated and indurated contents, has led to abrasion and loss of the walls of the part of the phragmocone covering it, and it seems to have been pressed downwards through one or two of the basal partitions deeper into the sheath than was natural, or than it is situated in other specimens. The capsule of the phragmocone extends about one-third of an inch beyond the ink-bag, and terminates by a well-defined border. The smooth surface of its opake white external calcareous layer is well preserved over nearly the whole of this part. The muscular tunic of the mantle (d) appears to commence at the peristome; it seems to have first undergone the change into adipocire and then to have become so brittle as to crack and break instead of bending to the pressure; the course of the muscular fibres is plainly visible; all those on the outer surface of the mantle which is presented to the observer, are transverse or circular; this surface is smooth, and the course of the fibres more feebly indicated; in the few places where the upper side of the mantle is broken away, and the inner surface of the opposite side shown, the arrangement of the transverse fasciculi is more strongly displayed.

In the length as compared with the breadth of the mantle, the Belemnite is shown by this beautiful specimen to have had the same elongated form of body as the Onychoteuthis and most modern Decapoda.

* Oken's Isis, Bd. xxi. p. 438.

MDCCCXLIV.

L

↑ PL. III.

A little above the capsule of the phragmocone, on the left side, there is a flattened transversely fibrous body (e) with a rounded external border, so well defined as to excite the suspicion that it must have belonged to some part superadded to the muscular mantle; its nature is demonstrated in the specimen next to be described.

An oblong portion of the same fibrous and muscular tissue as the mantle, lying obliquely in front of the anterior margin of the mantle, and in which both a longitudinal and transverse layer of fibres are discernible, seems to be the remains of the infundibulum or expiratory tube (ƒ).

The direction of the fibres in the cephalic arms is chiefly longitudinal; the magnified figure* precludes the necessity of describing the shape of the horny hooks; their arrangement in a double alternate series is manifested in some parts of the arms, but is still more obvious in the third specimen to be described.

The second less complete but highly instructive specimen of the Belemnite is from the collection of Mr. PRATT; it exhibits part of the muscular mantle (d), the two fins (e, e), apparently the infundibulum (ƒ), the ink-bag and duct (n), and a considerable proportion of the phragmocone (c). This part is more distorted and less entire than in the preceding specimen, but, so far as a comparison can be made, presents the same form and structure §.

The reservoir of ink is situated two lines within the aperture of the phragmoconic capsule, which terminates with the same well-defined border; it is of an oval form and jet black colour, with a feeble indication of its original nacreous outer coating: the inspissated ink is very hard, brittle and splintering; when reduced to a fine powder it presents a dark brown hue, and, used as a pigment, works as smoothly as Roman Sepia, but with a deeper tint.

The Belemnitic ink-secretion offers the closest resemblance with that described by Dr. BUCKLAND from the lias of Lyme Regis, and which he found associated with a series of circular transverse plates and narrow chambers, resembling the chambered cone within the alveolus of a Belemnite, and from which Dr. BUCKLAND inferred that the animal, from which these fossil ink-bags were derived, was some unknown Cephalopod, nearly allied in its internal structure to the Belemnite, the circular form of the septa showing that they could not be referred to the molluscous inhabitant of any Nautilus or Ammonite||.

The absence of the ink-apparatus in the Nautilus and allied extinct chambered Cephalopods adds demonstrative proof, were such required, of the accuracy of Dr. BUCKLAND'S negation, whilst the association of the spathose guards with crushed phragmocones, identical in structure with those in connection with the fossil ink-bag and muscular parts of the specimens under consideration, and all from the same * Pl. IV. fig. 2. Pl. IV. fig. 1.

† Pl. V.

§ Near the confused remains of the head in this specimen there is the impression of part of the phragmocone of another Belemnite.

Philosophical Magazine, 1829.