Of the ten-footed section of the cephalopods, there are four “families”; two only of which exist in Britain—the Teuthidæ, and the Sepiidæ. The Teuthidæ are the Squids, or Calamaries, represented by the long-bodied _Loligo vulgaris_, that has along its back a gristly, translucent stiffener, shaped like a quill pen; from which and its ink it derives its names of “calamary,” “pen-and-ink fish,” and “sea-clerk.” The Sepiidæ are the Cuttles; as a type of which we may take the common “cuttle-fish,” _Sepia officinalis_, the owner of the hard, calcareous shell often thrown up on the shore, and known as “cuttle-bone,” or “sea-biscuit.” Of the eight-footed cephalopods,—the Octopoda,—there are two families; namely, the Octopidæ, and the Argonautidæ. The first only is found on our coasts. The British members of it are the common Octopus, _O. vulgaris_, and the Eledone, _E. cirrosa_, a genus chiefly distinguished from the octopus by its having only one row of suckers, instead of two, along its arms or feet. The Argonautidæ, which inhabit warmer seas than ours, and approach no nearer to us than the Mediterranean and Adriatic, are represented by _Argonauta argo_, the “Paper Nautilus,”—so called from the peculiar texture of its shell, and the similarity of its shape to that of the true Nautilus, _N. pompilius_, from which, however, it differs greatly in organisation. All of these four “families” have two plume-like gills,—one on each side—and are therefore placed by Professor Owen in the “order,” Dibranchiata. To this order belong also the extinct Belemnites, and the still living Spirula, only one entire specimen of which has ever been obtained, and that was in New Zealand, though its beautiful internal shells are sometimes thrown up on the shores of Devon and Cornwall. The Tetrabranchiata, or four-gilled cephalopods, are represented by a single living genus—the Pearly Nautilus, _N. pompilius_,—but in Silurian times by 34 genera, and more than 1400 species.[5] The following diagram will help to explain the relationship of the Octopus to the rest of the cephalopoda.