They appear, in fact, to consist of nothing but concentric layers of carbonate of lime, disposed in subcrystalline fibres, or prisms, perpendicular to the layers. Among a great number of specimens of these Belemnites, however, it was soon observed that some showed a conical cavity at the blunt end; and, in still better preserved specimens, this cavity appeared to be divided into chambers by delicate saucer-shaped partitions, situated at regular intervals one above the other. Now there is no mineral body which presents any structure comparable to this, and the conclusion suggested itself that the Belemnites must be the effects of causes other than those which are at work in inorganic nature. On close examination, the saucer-shaped partitions were proved to be all perforated at one point, and the perforations being situated exactly in the same line, the chambers were seen to be traversed by a canal, or _siphuncle_, which thus connected the smallest or apical chamber with the largest. There is nothing like this in the vegetable world; but an exactly corresponding structure is met with in the shells of two kinds of existing animals, the pearly _Nautilus_ and the _Spirula_, and only in them. These animals belong to the same division—the _Cephalopoda_—as the cuttle-fish, the squid, and the octopus. But they are the only existing members of the group which possess chambered, siphunculated shells; and it is utterly impossible to trace any physiological connection between the very peculiar structural characters of a cephalopod and the presence of a chambered shell. In fact, the squid has, instead of any such shell, a horny “pen,” the cuttle-fish has the so-called “cuttle-bone,” and the octopus has no shell, or, at most, a mere rudiment of one. Nevertheless, seeing that there is nothing in nature at all like the chambered shell of the Belemnite, except the shells of the _Nautilus_ and of the _Spirula_, it was legitimate to prophesy that the animal from which the fossil proceeded must have belonged to the group of the _Cephalopoda_. _Nautilus_ and _Spirula_ are both very rare animals, but the progress of investigation brought to light the singular fact, that, though each has the characteristic cephalopodous organisation, it is very different from the other. The shell of _Nautilus_ is external, that of _Spirula_ internal; _Nautilus_ has four gills, _Spirula_ two; _Nautilus_ has multitudinous tentacles, _Spirula_ has only ten arms beset with horny rimmed suckers; _Spirula_, like the squids and cuttle-fishes, which it closely resembles, has a bag of ink which it squirts out to cover its retreat when alarmed; _Nautilus_ has none.