Yonder Thorny Woodcock (_Murex tenuispina_) is a still more striking shell than either, and one whose periodic growths are peculiarly well marked. It is covered at regular intervals with rows of shelly spines, still longer and more numerous than those we lately admired in the _Dione_. Each series crowns a thickened ridge, which runs across the whorl, as regards the direction of its growth, but longitudinally as regards the general figure of the shell. Now, the increase of the shell in the Univalves is performed almost exactly as in the Bivalves; namely, by the protrusion and eversion of the mantle on the existing edge. And, therefore, each of these thorny ridges, separated as they are by an interval of just two-thirds of a whorl, marks the termination of a new growth, the shelly matter rising up at the margin in this thickened ridge, which bristles with elongated points. In this specimen we can trace ten such ridges, whence we legitimately infer ten distinct periods through which this animal has passed, besides the nautiloid stage under which all the creatures of this Class commence existence. Yet, since each of these three univalves has been this day created, these inferences are deceptive. The Scorpion-shell was _never_ otherwise than dilated and digitated. The Cowry has _never_ had a lip that was not thickened, nor an exterior that was not porcellanous. The Woodcock has _never_ known a moment in which its thorns were less numerous than they are now. Notice that fine round shell carried along the floor of the sea, by means of a great fleshy tortoiseshell-coloured[75] body, which, with a head of many spreading tentacles applied to the ground, crawls with a tolerably quick progress.[76] It is the Pearly Nautilus. The amplitude of the beautiful nacreous shell is by no means a measure of the dimensions of the animal; for this merely sits within the shallow mouth, like a Welsh fisherman in his coracle. If we remove the creature, we shall find the cavity bounded by a pearly floor, in the centre of which is a slender tube running down from it. On breaking away this floor, we expose an empty chamber, with a similar pearly floor, through which passes the shelly tube, continued through the middle of the chamber, and running down to the next. Thus we should find the whole interior of the shell occupied by a series of these empty chambers, fifty or upwards in number, each less than its predecessor (rather _successor_, if we regard them in the order of development), until we can trace them no longer in the minute centre of the spire.