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a double keel-shaped indented process, enlarging from its apex backwards.* (Original.)

PLATE 32. V. I. p. 244.

Fig. 1. Part of the petrified shell, and casts of the interior of some of the chambers, of a Nautilus hexagonus, from Marcham, Berks. This fossil exhibits at its smaller End, from d to b., a series of casts of the Air chambers, from which the external shell has been removed. The cavity of each chamber is filled with a disc of pure calcareous spar, representing the exact form of the chamber into which it had been infiltrated. In the larger portion of this fossil, the petrified shell retains its natural place, and exhibits fine wavy lines of growth forming minute Ribs across its surface. (Original.)

Fig. 2. Fractured shell of N. hexagonus, from the Calca

reous grit of Marcham. The chambers are lined
with calcarcous spar, and a circular plate of the
same spar is crystallized around the siphon. The
interior of the siphon is filled with a cast of Calcare-
ous grit, similar to that which forms the rock from
which the shell was taken. See V.
See V. I. p. 247.† (Ori-
ginal.)

Although the resemblances between these fossil beaks, and that of the animal inhabiting the N. Pompilius, are such as to leave no doubt that Rhyncholites are derived from some kind or other of Cephalopod, yet, as they are found insulated in strata of Muschel kalk and Lias, wherein there occur also the remains of Sepiæ that had no external shells, we have not yet sufficient evidence to enable us to distinguish between the Rhyncholites derived from naked Sepiæ, and those from Cephalopods that were connected with chambered shells. I possess a specimen of a fossil Nautilus from the Lias at Lyme Regis, in which the external open chamber contains a Rhyncholite.

This fossil exhibits the Siphuncle in its proper place, passing

Fig. 3. represents in its natural size, a portion of the Siphuncle which in Fig. 2. is laid bare along its course through the chambers, d. e. f. In the transverse Plate, h, the siphuncular collar is entire, but a Section of another collar in the transverse Plate, i, shows the contraction of the Siphon at its passage through this aperture, and exhibits also the overlapping, or squamous suture by which the Collar is fitted to the superior and inferior portions of the calcareous Sheath of the Siphon. See V. I. pp. 247, 248. Note. (Original.)

A similar structure may be seen at the Collars of the transverse Plates of the N. Striatus. See Pl. 33.

across the cavities of the Air chambers. As in the recent Nautilus Pompilius, there is no communication between the interior of the Siphon and that of the Air chambers, so in this fossil shell, there is proof that no communication existed between these cavities. A transverse section at a. shows the thin edge of the sheath of the siphuncle, surrounded externally with calcareous spar, and filled internally with Grit. Other Sections of the Siphuncle at b. d. e. f. show the calcareous Grit within its cavities to be contracted at its passage through the collars of the transverse plates, and most enlarged midway between one transverse plate and another.

This fossil affords two proofs that no communication existed between the interior of the Siphuncle and that of the Air chambers. 1st. the calcareous sheath of the Siphuncle is seen at d. e. f. completely enclosing the calcareous grit which forms the cast within it. 2dly, had there been any communication between the interior of the siphuncle, and that of the air chambers, these chambers must have received some portion of the materials of the grit that have filled this Siphuncle: not a particle of grit is found in any one of the adjacent air chambers, but they are all lined, and some of them nearly filled with a crystalline deposite of Carbonate of Lime, disposed in uniform plates around the.. interior of each chamber, and around the Siphuncle. See Fig. 2. c. c1, a. a1, a3, a3. and Fig. 3. d-k. This deposite can only have been formed from water charged with carbonate of lime, introduced by infiltration, after the interment of the shell, and filling the chambers which are thus uniformly invested.

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PLATE 33. V. I. pp. 247, 248. Note.

Longitudinal Section of Nautilus Striatus, from the Lias at Whitby, in the collection of Mrs. Murchison. The interior of the Chambers is filled exclusively with calcareous spar, and that of the Siphuncle with Lias. (Original.)

a. The Siphuncle: the union of the siphuncular calcareous sheaths, with the aperture or collar of each transverse Plate, is so closely fitted, that no fluid could have passed between them into the air chambers. b. One of the transverse Plates forming the Air chambers. c. White calcareous spar, filling the middle region only of the air chambers.

d. Stratified zones of dark coloured calcareous spar, deposited in equal thickness on both sides of the transverse plates, and also on the inside of the shell, and around the calcareous sheath of the siphuncle.* e. Portion of the external, shell, showing a laminated

structure.

PLATE 34. V. I. p. 249. Note.

Drawing of the animal of the Nautilus Pompilius, prepared at my request by Mr. Owen, to show the manner in which the siphuncle terminates in the Pericardium. (Original.)

* The successive zones of this dark Spar show that the Lime composing it was introduced by slow and gradual infiltrations into the cavity of the air chambers. Hence it follows that no communication existed between the Siphuncle and these chambers, at the time when this Pipe was filled with the fluid mud, that has formed a cast of Lias within it. As the fractures across the Siphuncle in the 2d and 3d chambers are filled only with spar, of the same kind as that within these Chambers, these fractures could not have existed, when the Mud of the Lias formation entered the Siphuncle, without admitting it also into the chambers adjacent to them.

a. The Heart.

b. A bristle passing from the pericardium through the membranous siphuncle laid bare.

c. Bristles passing from the pericardium through the orifices of communication with the Branchial chamber.

d. d. d. d. Follicles communicating with the Branchial Arteries.*

'd. 'd. 'd. 'd. Pericardial septa, forming thin muscular Receptacles of the follicles.

e. e. The Branchiæ.

f. The Branchial Chamber.

g. The Funnel, or Branchial outlet.

h. The infundibular valve.

i. i. The digital processes.

k. The Gizzard.

7. The Ovary.

m. m. The mantle dissected off.

n. The membranous siphuncle.

0. o. The siphuncular artery.

p. p. The Boundaries of the Pericardial cavity.

q. Portion of the Siphuncle between the Pericardium and first transverse plate of the shell.†

* Mr. Owen supposes that these follicles discharge the impurities of the blood into the Pericardium, when there is no access of water to the Branchiæ, during the time that the animal is contracted within its shell. The overflowings of this pericardial fluid may pass out through the orifices marked by the bristles, c. c.

This upper portion or neck of the Siphuncle, has the form of a flattened canal, with thin Parietes of the same substance as the Pericardium; when the animal expands itself at the bottom of the sea, this neck is probably closed by the lateral pressure of the gizzard, k, and ovary, l, and so acts instead of a valve to prevent the return of the pericardial fluid into the Siphuncle. At such times, the deep-sea water must press with great force on the exterior of the Pericardium, and tend to force the pericardial fluid into the Siphuncle; but as an equal amount of pressure is applied simultaneously to the Ovary and

PLATE 35. V. I. p. 257.

Cast of the interior of the Shell of Ammonites obtusus from Lyme. Fragments of the shell remain near b. and e.

One object of this Plate and of many of the figures at Pl. 37. is to show the manner in which the external shell is fortified by Ribs and Flutings, (PP. 257. 258.) and further supported by the edges of the internal transverse plates, that form the air-chambers. See V. I. p. 263, Note. (Original.)

PLATE 36. V. I. p. 256. Note.

Longitudinal section of another shell of Ammonites obtusus from the Lias at Lyme Regis. (Original.)

The greater part of the outer chamber, and the entire cavities of the air-chambers are filled with calcareous spar, and the Siphuncle, (preserved in a carbonaceous state,) is seen passing along the entire dorsal margin to the commencement of the outer chamber. See V. I. p. 265, Note.

Von Buch has found evidence to show that the membranous siphuncle of Ammonites was continued to a considerable distance along the outer chamber, beyond the last or largest transverse Plate. This discovery accords with the analogies afforded by the membranous neck of the siphon of the N. Pompilius, which is continued along the outer chamber from the last transverse Plate to the Pericardium. See Pl. 34. q.*

Gizzard, the lateral pressure of these two organs on the neck of the Siphuncle would tend to close it with a force exactly counterbalancing the external pressure on the Pericardium.

* As the body of the animals that inhabited the Ammonites was more elongated than that of those inhabiting of the shells of Nautili, in consequence of the smaller Diameter of their outer Chamber, the place of their Heart was probably more distant from the last transverse Plate, than that of the Heart of Nautili; and the membranous Siphon connected with the Pericardium consequently longer.

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