genus-viz.: Nautilus, remain, although in the earlier periods of the earth's history they were most abundant -more so perhaps than any other family of the class Mollusca.

FIG. 212.

The structure of the internal cartilaginous skeleton of one of the commonest members of the Di-branchiate order, the Cuttle-fish, Sepia officinalis, has already been described in the first volume of these Lectures; I shall therefore only briefly allude to it again, as exhibiting cells of a peculiar figure, somewhat like the bone cells of many fishes: they are imbedded in a semitransparent matrix, as shown in Fig. 212; the same structure exists in the cartilage of most of the soft Cephalopods, but in some species a fibrous tissue is mixed up with the cartilage.

Cartilage from the cranium of Sepia officinalis.

I shall now proceed to describe the structure of the rudimentary shell in some of these animals, and will first take that of the Octopus, or Poulpe, in which it exists simply as a horny Style. When this is divided transversely, it is found to consist of concentric laminæ of brown horny material; in some specimens the centre of the section is much more consolidated and of a darker colour than the outside. The Pen or Gladius of the Calamary, Loligo vulgaris, as shown in Fig. 213, is very like a quill in shape; when divided transversely it also exhibits a laminated structure like that of the Octopus, but when that

FIG. 213.

part which corresponds to the plume of the quill is carefully examined, a very faint trace of hexagonal cells, like those shown in Fig. 214, has been observed; from these markings not being noticed in the transverse sections, I am induced to believe that they belong to a cuticular layer, and do not enter further into the structure of the horny gladius.

In the Cuttle-fish, Sepia officinalis, the

bone," or Sepiostaire, as shown in Fig. 215, is usually considered as a rudiment of a chambered shell, one chamber of which, and this the last or outer one, has been developed unilaterally. The "bone" is usually as long and as wide as the mantle in which it is imbedded; the outer surface is convex and shelly, the inner concave and chalky; on the edges is a brown horny substance which is very broad at the lower portion of the bone, and in the median line terminates in a pointed process termed the mucro. In former times this rudimentary shell was employed in medicine as an "antacid," but its principal use now, is either for tooth-powder or for "pounce." The outer shelly portion of the Sepiostaire, when rendered sufficiently thin to be transparent, exhibits a coarse hexagonal structure, as shown at F in Fig. 216; each hexagon has an oval nucleus in the centre, from

The Hornypen or Gladius of a Calamary, Loligo vulga

ris.

which a series of radii proceed towards the margin;

FIG. 215.

the tooth of Mya arenaria, Fig. 187. The soft internal portion of the same specimen exhibits a totally different structure. On transverse section, as shown at D, in Fig. 216, it will be found to be composed

of a series of curved laminæ, a a, separated from each other by thin wavy partitions, or pillars, of transparent calcareous material, each of which, when highly magnified, as shown at E, exhibits a series of fine transverse markings, somewhat like those seen in the prisms of the Pinna. In consequence of the laminæ being curved, those portions of the pillars which are attached to the concave side of each, are much broader than those attached to the convex side, as may be readily seen on referring to Fig. 216, D. When a portion of the Sepiostaire is divided longitudinally, the

The Sepiostaire,

or cuttle-bone of pillars will be found to project from the laminæ, and being viewed edgeways,

Sepia officinalis.

their true nature can be easily ascertained; they may

therefore be described as consisting of a thin layer of calcareous material folded or bent upon itself like a

D, a portion of a transverse section of Cuttle-bone; a a, laminae between the pillars. E, one of the pillars magnified 95 diameters. F, portion of the outer shelly substance of the Cuttle-bone. G, concave inner surface of one of the laminæ.

frill. When separated from the laminæ, they leave an impression of their exact shape upon them, as shown at G, which is a representation of one of the laminæ having its concave surface uppermost. It will thus be seen that there must be a considerable part of each lamina not occupied by the pillars; all these spaces therefore may be considered as so many cavities in, and as diminishing considerably the weight of, the mass. The structure of the laminæ is minutely granular, and the calcareous material of which it is composed is far more opaque than that composing the pillars.

LECTURE XXI.

SKELETON OF MOLLUSCA-CEPHALOPODA.

THE Argonaut, or Paper-sailor, belongs to the same order as the Cuttle-fish. The animal very much resembles the Octopus in shape; it has eight arms, two of which are provided with expanded extremities, and perform the function of a mantle in secreting and protecting the delicate shell. The Argonaut was formerly described as having the power of coming to the surface of the sea in fine weather, and spreading its thin sail to catch the breeze, but I need hardly tell you that this is not the case; it progresses as the Octopus does, by ejecting the water from its funnel; it also is able to crawl over rocks and stones by means of its arms, the shell being reversed like that of the Snail. The attachment of the animal to the shell is very slight, and when preserved in spirit, unless