The genera which belong to the second order are only two in number, viz. Sepia and Loligo. In both, the suckers on the arms are pedunculated, and their margin strengthened by a corneous ring furnished with teeth, and the margin of the anus is surrounded with appen dages or tentacula. The two feet with which the animals of this order are furnished are nearly similar in their structure to the arms, but considerably larger in their dimensions. They take their rise on the ventral side of the mouth, between that organ and the funnel.
4. Sepia. In this restricted genus the sac is furnished on each side throughout its whole length with a narrow fin, and strengthened on the back by a complicated cal careous plate, lodged in a peculiar cavity. The suckers are irregularly scattered on the arms and feet.
The calcareous plate has been long known under the name of cuttlefish bone. It is somewhat ovate, flatly convex on both sides, and thickest where broadest. The superior half, or the one next the head, is the longest, rounded at the extremity, and thin. The inferior por tion becomes suddenly narrow, and ends in a point. It may be considered as consisting of a dermal plate, concave on the central aspect, having its concavity filled up with layers which are convex on their central aspect.
According to our observations the dermal plate ap pears to consist of three different lamina:, arranged pa rallel to one another. The external or dorsal layer is rough on the surface, and marked by obscure concentric arches towards the summit, formed by minute knobs, which become larger towards the base, where they ap pear in the form of interrupted transverse ridges. It is uniform in its structure, and the tubercles possess re polish and hardness equal to porcellaneous shells, although they blacken speedily when put in the fire, and contain a good deal of animal matter. On the central side of this layer is one flexible and transparent, similar to horn, and smooth on the surface. The third layer is destitute of lustre, and in hardness and structure resembles mo ther of pearl shells.
The layers which fill the concavity of this dermal plate arc slightly convex on the central aspect, and are in part imbricated. Each layer is attached to the con cave surface of the dermal plate by the upper extremity and the two sides, while the inferior or caudal extremity is free. The inferior and first formed layers are short,
and occupy the base and middle, and rise from the plate under a more obtuse angle than the new formed layers, which are both the longest and the broadest.
Each layer, which is about one-fiftieth of an inch in diameter, consists of a very thin plate, whose dermal surface, when viewed with a magnifier, exhibits nume rous brain•like gyrations. From the ventral surface of this plate arise numerous perpendicular lamina:, which, when viewed laterally, appear like fine parallel threads, but, when examined vertically, are found to be waved, and fold upon themselves. Next the plate they are thin, and not much folded ; but towards their other extremity, they become thicket., striated across, and more folded, with irregular margins. On the thick tortuous even ends of these laminaz, the succeeding plate rests, and derives from them the peculiar markings of its surface. These lamina are closely set, irregularly interrupted, and occasionally anastontose. NI. Cuvier (Mem. Sur la Seiche, p. 47.) states, erroneously, that these lamina: are hollow pillars disposed in a quincunx order.
The term hone has been improperly applied to this complicated plate ; « for," (according to Mr. Hatchett, Phil. Trans. vol. lxxxix. p. 321.) " this substance, in composition, is exactly similar to shell, and consists of various membranes hardened by carbonate of lime, with out the smallest mixture of phosph•te." This bone was formerly much prized in medicine as an absorbent, but is now chiefly sought after for the purpose of polishing the softer metals.
The most remarkable species of this genus is the Sepia ofticinalis, which is distinguished from the others by its smooth skin. It inhabits the British seas, and, although seldom taken, its bone is cast ashore on dif ferent pal is of the coast, from the south of England to the Zeeland isles.
5. Loligo, or Calamary. This genus, the larger spe cies of which are known to the fishermen under the de nominations Sleeve-fish, Hose-fish, and Anchor-fish, is distinguished front the former by having the sides of the sac furnished only partially with fins. The dorsal plate is flexible and corneous, imbedded in the substance of the sac, and is multiplied with years. The suckers are disposed on the arms and feet in a double row.