"The common _cuttle-fish_, a native of our seas, will give you an idea of a cephalopode. "In the first family a fossil-shell occurs that is very frequently met with. "Family, _Orthocerata_; genus, _Belemnite_, thunder-stone, or arrow-head. These fossil-shells occur abundantly in the chalk formations. Many superstitious notions have been attached to this extinct marine animal: of these you will find an account in the 'Penny Cyclopædia.' "In the family _Lituolita_ we meet with the delicate and remarkable little shell _Spirula Peronii_, distinct from _Nautilus_ by the separation of the volutions: it is nearly covered by the body of the animal. The colour is white; the texture thin and brittle: it has a lateral syphon, the orifice of which is very clearly to be seen as each compartment is taken off. There is but one species; it is found in the Southern Ocean and the Moluccas. The shells are seen floating on the surface of the water when the animal is dead, and are sometimes carried to the shore. (Plate 9.) [Sidenote: AMMONITE. NAUTILUS.] "Another family of this order, _Nautilacea_, contains, among many other genera, the celebrated _Nautilus_. This genus, you are already informed, is distinguished from _Argonauta_ by its shells being many-chambered. Two or three fossil species have been found in London clay. "Among the various fossil-shells abounding in different strata, not known in a recent state, the one most remarkable and frequently occurring is the Ammonite, _Cornu Ammònis_, so called from the resemblance it bears to the convoluted horns of Jupiter Ammon, in mythological history. This is your _snake-stone_, Lucy, a local name, which you had better change for Ammonite. Various legends are connected with this fossil, of which you may obtain information by consulting the before-mentioned publication. "As you are desirous of studying geology, a knowledge of the Ammonites is very requisite, since whole sections of the genus are characteristic of certain strata. "They are nearly allied to the _Nautilus_. The species are very numerous; one hundred and twenty according to some authors--two hundred and seventy species are enumerated by others. They occur in Europe, Asia, and America: they have been found in the chalk with a diameter of three feet. "In the second division of this order the genus _Argonauta_ occurs: the shell has been already described, and retains its original name. (Plate 6.) [Sidenote: OCTOPUS. LOLIGO. SEPIA.] "In the third division is the family _Sepiaria_, containing _Octopus_, _Lolìgo_, and _Sepia_. This section contains animals without shells. _Octopus vulgàris_ is common in the European seas. In hot climates it grows to a very large size. The animal has sufficient strength to draw a boat under water.