of enabling the animal either to sink or float at pleasure; the common opinion, even now, is that the

FIG. 226.

animal has the power

Vertical section of the shell of Nautilus

of injecting water into the siphuncular tube, by which the specific gravity of the shell is increased, and it gradually sinks to the bottom; at the will of the animal, the water can be forced out of the siphuncle

pompilius, showing the siphuncle, a a, and the by the compressed air of the chambers, as

situation of the animal in the last chamber.

in the philosophical toy named the Hydrostatic paradox, and the shell, being thus rendered lighter than the water, again comes to the surface. If such a theory as this were applicable to the Nautilus, it certainly could not be to other shells found in the fossil state-as for instance, that shown in Fig. 228, the siphuncle of which is entirely composed of shelly matter; hence the idea entertained by Professor Owen of the use of the siphuncle, is, no doubt, the correct one.

We now proceed to the examination of the skeleton of the Nautilus pompilius, which we shall find well deserves the epithet pearly. The shell itself is so well known that I only think it necessary to point out that there are three distinct varieties of colour on its outer A A

VOL. II.

FIG. 227.
b

a

surface, each forming a very thin layer, the greater part being of an opaque white, the others of a reddishbrown and black. When divided vertically, as shown in Fig. 227, the greater portion of the section, as seen at a, consists of nacreous material, whilst the superficial layer, b, exhibits more or less of a cellular structure, and when magnified 500 diameters, presents the appearance represented at c. If a fragment of the nacreous lining be decalcified, it will

Vertical section of shell of Nautilus pompilius. a, nacreous layer. b, superficial layer. c, portion of superficial layer magnified 500 dia

meters.

show perhaps quite as plainly

as any other shell, that this

beautiful material is made up of cells.

Of the structure of the remaining members of the Tetrabranchiate order I have very little to say, as they are only known to us by their fossil remains. They were, however, so numerous in the paleozoic and secondary strata, that they appear to have been the principal representations of the carnivorous Mollusca at that period of the earth's history.

The type of the shell in all the fossil species is that of a cone, and this may be either straight or more or less convoluted, as in the Ammonite; between these two forms a great many varieties occur, and the shells in consequence have been named, Orthoceratites, Baculites, Turrulites, Hamites, &c., but whatever the

FIG. 228.

shape, every variety is characterized by possessing a series of septa, by which the shell is divided into a number of chambers, and the last is occupied by the animal. The outer shell is most frequently absent, and the curious zig-zag markings observable on the exterior of Ammonites, are the edges of the septa. In the Nautilus pompilius, as shown in Fig. 226, the septa are concave; this is their condition also in the Orthoceratite represented in Fig. 228. During life, the chambers, like those of the Nautilus, are empty, but in most fossil specimens, as may be seen in horizontal sections of Ammonites, they are filled with sparry crystals. Of the structure of the shell of these remarkable fossils I have little to say; the nacreous layer of the Ammonite, which I frequently find to be the only one left, exhibits precisely the same characters as that in the recent Nautilus, but the Ammonites are interesting in another point of view, as we have abundant evidence to prove that the mouth of the shell was more or less closed by an operculum either composed of horn or of shelly material. According to Woodward, in one group (Arietes) the operculum consists of a single piece, and is horny and flexible, but in others it is shelly and divided into two

Orthoceras gi

gantum, carbon

iferous limestone

of Britain (after Woodward).

plates by a median suture, the external

1

surface being smooth, the internal showing the lines of growth. These bodies were called Trigonellites by Parkinson in 1811, they have since been described by Meyer as bivalve shells under the generic name of Aptychus. M. Deshayes believes them to be calcareous plates belonging to the gizzard of the animal, but the prevailing opinion in this country is that they are opercula. I am indebted to Mr. Woodward for the specimen I now show you. When sections are examined microscopically, the shell-structure is very evident. The Aptychi are found either in the last chamber of the Ammonite, or in the matrix near to it. Mr. Moore of Ilminster has a very large collection of them, and to a paper of his, lately published in the "Transactions of the Somersetshire Archæological and Natural History Society" I would beg to refer you for much valuable information concerning them.

LECTURE XXII.

SKELETON OF MOLLUSCA-PEARLS.

BEFORE leaving the structure of the skeleton of the Mollusca, there are a few species which offer such peculiarities either in their habits or their internal anatomy as to require a brief notice. Many of the bivalve Mollusca are "boring" animals-of these the most remarkable are the Pholas and Teredo; the first, Fig. 229, bores in wood or clay, and two principal

liquid is excreted which acts as a solvent. The view