Page:Natural History, Mollusca.djvu/76

64 Most of the species of the Class are destitute of any external covering, but a few are protected by a shell. An immense number of fossil species, however, belonged to the latter division, known by the names of Belemnite, Nummulites, Orbulites, Ammonites, &c. The most important existing species possessing a shell, are te Pearly Nautilus of the tropical seas, and the Paper Nautilus of the Mediterranean. The former of these possesses four gills, a peculiarity which distinguishes it from all its fellows: Professor Owen, therefore, proposes to divide the Class into two Orders; the one including the Pearly Nautilus, called Tetrabranchiata, or four-gilled; the other including the rest of the existing species, named Dibranchiata, or two-gilled.

As the animals belonging to the Molluscous Division are very numerous, I propose to limit this volume to those families which are represented by species existing in and around the British Islands, contenting myself with an occasional slight notice of such foreign kinds as have anything particularly interesting in their history. I shall therefore at once proceed to the second of these Orders.