Family 30.—HELICINIDÆ. (7 Genera, 433 Species.) (1868.)
| General Distribution. | |||||
 | |||||
| Neotropical Sub-regions. | Nearctic Sub-regions. | Palæarctic Sub-regions. | Ethiopian Sub-regions. | Oriental Sub-regions. | Australian Sub-regions. |
| — 2. 3. 4 | — — 3 — | — — — — | — — — — | — — 3. 4 | 1. 2. 3 — |
The Helicinidæ are very characteristic of the Antilles, comparatively few being found in any other part of the world except the Islands of the Pacific. The genera are:—
Trochatella (33 sp.), Antilles with a species in Venezuela, and another in Cambodja; Lucidella (5 sp.), Antilles; Helicina (274 sp.), Antilles, Pacific Islands, Tropical America, Southern United States, Moluccas, Australia, Philippines, Java, Andaman Islands and North China; Schasicheila (5 sp.), Mexico, Guatemala and Bahamas; Alcadia (28 sp.), Antilles; Georissa (5 sp.) Moulmein to Burmah. About 10 per cent. of new species appear to have been since described in the larger genera of this family.
General Observations on the Distribution of the Land Mollusca.
A consideration of the distribution of the families and genera of land-shells shows us, that although they possess some special features, yet they agree in many respects with the higher animals in their limitation by great natural barriers, such as oceans, deserts, mountain ranges, and climatal zones. A remarkable point in the distribution of these animals, is the number of genera which have a very limited range, and also the prevalence of genera having species scattered, as it were at random, all over the earth. No less than 14 genera (or about one-sixth of the whole number) are confined to the Antilles, while the greater part of the sub-genera of modern authors are restricted to limited areas.
If we first compare the New World with the Old, we find the difference as regards genera quite as great as in most of the vertebrates. In the Helicidæ, 10 genera are confined to the New, and 7 to the Old World, 16 being common to both. In the Operculata the number of genera of restricted range is greater,—the New World having 15, the Old World 32 genera, only 8 being common to both. Of the New World genera 12 out of the 15 do not occur at all in South America; and of those of the Old World, 22 out of the 32 occur in a single region only. If we take the northern and southern division proposed by Professor Huxley (the latter comprising the Australian and Neotropical regions), we find a much less well-marked diversity. Among the Helicidæ only 4 are exclusively northern, 8 southern; while among the Operculata 22 are northern, 16 southern. The best way to compare these two kinds of primary division will be to leave out all those genera confined to a single region each, and to take account only of those characteristic of two or more of the combined regions; which will evidently show which division is the most natural one for this group. The result is as follows:—
Genera common to two or more Regions in, and confined to, each Primary Division of the Earth.
| Helicidæ | Operculata. | Totals. | |||
 | Northern | 0 | 0 | 0 |  |
| Southern | 0 | 0 | 0 | ||
| Old World | 1 | 12 | 13 | |
| New World | 4 | 0 | 4 | ||
We find then that the northern and southern division of the globe is not at all supported by the distribution of the terrestrial molluscs. It is indeed very remarkable, that the connection so apparent in many groups between Australia and South America is so scantily indicated here. The only facts supporting it seem to be, the occurrence of Geotrochus (a sub-genus of Helix) in Brazil, as well as in the Austro-Malayan and West Pacific Islands and North Australia; and of Bulimus in the same two parts of the globe, but peculiar sub-genera in each. But in neither case is there any affinity shown between the temperate portions of the two regions, so that we must probably trace this resemblance to some more ancient diffusion of types than that which led to the similarity of plants and insects. Still more curious is the entire absence of genera confined to, and characteristic of Africa and India. One small sub-genus of Helix, (Rachis), and one of Achatina, (Homorus), appear to have this distribution,—a fact of but little significance when we find another sub-genus of Helix, (Hapalus), common and confined to Guinea and the Philippine Islands; and when we consider the many other cases of scattered distribution which cannot be held to indicate any real connection between the countries implicated. No genus is confined to the Palæarctic and Nearctic regions as a whole. A large number of sub-genera, many of them of considerable extent, are peculiar to one or other of these regions, but only 3 sub-genera of Helix and 2 of Pupa are common and peculiar to the two combined, and these are always such as have an Arctic range and whose distribution therefore offers no difficulty.