The _Cephalopods_, or highest class of mollusca, are represented among the British carboniferous strata by seven genera. Of these the most characteristic is the _orthoceras_, so named from its shell being like a long straight horn. When the animal was young it inhabited a single-chambered shell like that of many of the gastropods, but as it increased in size and prolonged its shell in a straight line, it withdrew from the first occupied chamber. This was partitioned off by a thin wall called a _septum_, through the centre of which a tube ran to the narrow end of the shell (Fig. 27). As the creature grew, chamber after chamber was in this way formed, each of them quite air-tight, and traversed by the central tube. Suppose a graduated series of diminutive watch-glasses to be pierced by a long tapering glass-tube in such a way that they should have their convex faces towards the narrow end of the tube, and be arranged at short intervals, the smallest one placed near the point of the tube, and the largest a little below the wider end. Suppose, further, that this piece of mechanism were placed within another tube tapering to an obtuse point, and that the edges of the watch-glasses fitted tightly to the inner surface of this larger tube. Such would be a rough model of the structure of the orthoceras. The inner tube that traverses the centre of the chambers from end to end of the shell is called the _syphon_, but its uses are very problematical. At one time naturalists inclined to regard it as intended to be filled with fluid, which, by expanding the membrane of the tube, would compress the air in the chambers, and thus, increasing the specific gravity of the animal, enable it to sink to the bottom. In this way, by emptying or filling the syphonal tube, the orthoceras might have risen rapidly to the surface of the deep, or sunk as swiftly to the bottom. But this view, so pretty that one wishes it were confirmed, must be regarded as at least doubtful. The orthoceras more probably owed its power of progression to the action of a funnel connected with the breathing apparatus, whereby jets of water were squirted out that drove the shell rapidly along. The use of the air-tight chambers was, perhaps, to give buoyancy to the shell so as to make it nearly of the same specific gravity as water. Such a provision must have been amply needed, for Professor Owen mentions an orthoceras from Dumfries-shire that measured six feet in length, and similar gigantic specimens have been found in America. Unless the chambers in these shells had been air-tight, the animals that inhabited them would have been held down about as firmly to one spot as if they had been tied to a sheet-anchor. No mollusc could have possessed much locomotion with so ponderous a tail, six feet or more in length, to drag after it. But this inconvenience was obviated by the simple plan of having the chambers close, and filled with nitrogen or other gas evolved by the chemistry of the inmate.