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il2 GEOGRAPHICAL ZOOLOGY. [part iv.
Family 20.—DENTALIAD^. (1 Genus, 50 Species).
Distribution.—The genus Dentalmm is found in the North
Atlantic, Mediterranean, "West Indies and India. There are
125 fossil species, found in various formations as far back as
the Devonian in Europe and in Chili.
Family 21.—CHITONID^. (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 Molluscsr)
The Land and Fresh-water snails are so important and exten-
sive 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 Helicidse I follow the classical work of
Albers
—
Die Helicien, Von Martens' Edition (1860) ; and for
the Operculate families, Pfeiffer's Monographia Fneumonopomorum
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.—HELICID^. (33 Genera, 3,332 Species) (1860).
General Distribution.—Universal.
. The Helicidse, 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