Family 21.—CHITONIDÆ. (1 Genus, 250 Species).
Distribution.—On rocky shores in all parts of the world. There are 37 fossil species ranging back to the Silurian period.
Order II.—PULMONIFERA. ("Terrestrial Molluscs.")
The Land and Fresh-water snails are so important and extensive a group, and their classification has been so carefully studied, that their geographical distribution is a subject of much interest. The range of the genera will therefore be given in some detail. For the Helicidæ I follow the classical work of Albers—Die Helicien, Von Martens' Edition (1860); and for the Operculate families, Pfeiffer's Monographia Pneumonopomorum Viventium, 2nd Supplement, 1865. The number of species is, of course, very considerably increased since these works were published (and the probable amount of the increase I have in most cases indicated), but this does not materially affect the great features of their geographical distribution.
Family 22.—HELICIDÆ. (33 Genera, 3,332 Species) (1860).
General Distribution.—Universal.
The Helicidæ, or Snails, are a group of immense extent and absolutely cosmopolitan in their range, being found in the most barren deserts and on the smallest islands, all over the globe. They reach to near the line of perpetual snow on mountains, and to the limit of trees or even considerably beyond it, in the Arctic regions; but they are comparatively very scarce in all cold countries. The Antilles, the Philippine Islands, Equatorial America, and the Mediterranean sub-region are especially rich in this family. Comparatively few of the genera, and those generally small ones, are restricted to single regions; but on the other hand very few are generally distributed, only two—Helix and Pupa—occurring in all the six regions, while Helix alone is truly cosmopolitan, occurring in every sub-region, in every country, and perhaps in every island on the globe.
The Neotropical region is, on the whole, the richest in this family, the continental Equatorial districts producing an abundance of large and handsome species, while the Antilles are pre-eminent for the number of their peculiar forms. This region possesses 22 of the genera, and 6 of them are peculiar.
The Palæarctic region seems to come next in productiveness, but this may be partly owing to its having been so thoroughly explored. It possesses 16 of the genera, and 3 of them are confined to it. The great mass of the species are found in the warm and fertile countries surrounding the Mediterranean Sea.
The Ethiopian region has 13 genera, only one of which is peculiar.