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Oncidium (16 sp.), South Europe (1 sp. British), Mauritius, Australia and Pacific Islands; Vaginulus (20 sp.), Neotropical and Oriental regions.
FAMILY 25.—LIMNÆIDÆ (7 Genera, 332 Species.)
The Limnæidæ, or Fresh-water Snails, inhabit ponds and rivers in most parts of the world, but appear to be absent from the Australian region. The genera are distributed as follows :
Limnoa (95 sp.), Nearctic, Palæarctic, and Oriental regions ; Choanomphalos (2 sp.), Lake Baikal; Pompholyx (2 sp.), Western America; Chilinia (18 sp.), South America ; Physa (20 sp.), Nearctic, Palæarctic, Ethiopian and Oriental regions, and extends to above 73° North Latitude in Siberia, being the most Arctic of land or fresh-water shells; Ancylus (49 sp.), Nearctic and Neotropical regions, Europe and New Zealand; Planorbis (145 sp.), Nearctic, Palæarctic and Oriental regions. Several genera are found fossil, chiefly in the Wealden, Eocene, and Miocene formations.
FAMILY 26.--AURICULIDÆ. (3 Genera, 210 Species.)
The Auriculidæ are chiefly found near the sea in hot countries and are most abundant in the Eastern tropics. They are absent
from the East coast of South America. The genera have a somewhat restricted distribution as follows:
Auricula (128 sp.), India, Pacific Islands, Peru and West Indies; Melampus (56 sp.), West Indies and Europe; Carychium (9 sp.), Europe and North America ; Plectrotrema (14 sp.), Australia, Malay Islands, China, Cuba; Blauneria (2 sp.), West Indian and Sandwich Islands. There are many fossil species ranging back to the Eocene formation.
FAMILY 27.-ACICULIDÆ. (4 Genera, 65 Species.) (1865.)
The Aciculidæ are small cylindrical shells chiefly found in the West Indian Islands, but with representatives widely scattered over the globe.
Acicula (5 sp.) is European only; Geomelania (21 sp.), and Chittya (1 sp.), are confined to the Island of Jamaica ; Truncatella (38 sp.), is most abundant in the Antilles, but is also found in some part of each of the six regions, as indicated by the diagram of the family. But few new species have been added to this group.
FAMILY 28.—DIPLOMMATINIDÆ. (3 Genera, 23 Species.)
The Diplommatinidæ are minute shells of the Oriental and Australian regions.
Diplommatina (18 sp.) inhabits India to Burmah, and the greater part of the Australian region; the number of species has now been doubled, and one has been discovered in the island of Trinidad ; Clostophis (1 sp.), Moulmein; Paxillus (3 sp.), Borneo, Hong Kong, and Loo Choo Islands.
FAMILY 29.-CYCLOSTOMIDÆ. (41 Genera, 1009 Species.)
This extensive group, comprising the largest of the operculated land-shells, is especially characteristic of the Oriental region, which possesses 25 genera, no less than 12 of them being wholly confined to it. The Neotropical region comes next, with 15 genera, 9 of which are peculiar; but a large number of these are confined to the West Indian Islands, South America itself being very poor in this group.
The Palæarctic region has 3 peculiar genera; the Ethiopian and Australian 1 each. The Nearctic region has but a single West Indian species in Florida. The distribution of the genera is as follows :
Peculiar to or characteristic of the Oriental region are, Opisoporus (11 sp.), Rhiostoma (6 sp.), Alycaeus (39 sp.), Opisthostoma (1 sp.), Hybocistis (3 sp.), Pterocyclos (19 sp.), extending to the Moluccas ; Aulopoma (4 sp.), Dermatocera (4 sp.), Leptopoma (54 sp.), extending west to the Seychelles and east to the Moluccas and New Guinea ; Cyclophorus (163 sp.), most abundant in the Oriental region, but ranges to Japan, to Chili, and all Tropical America, over the whole Australian region, and to Natal and Madagascar, Cataulus (15 sp.), confined to Ceylon, the Neilgherries and Nicobar Islands; Rhaphaulus (4 sp.), Penang to Ceram; Streptaulus (1 sp.), Arinia (3 sp.), Pupinella (2 sp.), Pupina (24 sp.), half in North India to Philippines and
Japan, the other half in Moluccas, New Guinea and Australia ; Cyclotopsis (2 sp.), India and Malaya : Registoma (9 sp.), Philippines and Moluccas, New Caledonia and Pacific.
Characteristic of the Neotropical region are :-Cyclotus (111 sp.), half in the Antilles and Tropical America, the rest in the Moluccas, China, Malaya, India, Natal, and the Seychelle Islands; Megalomastoma (27 sp.), abundant in Cuba, West Indies and South America, others in India, Malaya, and Mauritius; Jamaicia (2 sp.), Jamaica; Licina (5 sp.), Antilles ; Choanopoma (49 sp.), Antilles; Ctenopoma (25 sp.), Antilles ; Diplopoma (1 sp.), Cuba; Adamsiella (15 sp.), Jamaica, Cuba, Guatemala ; Cyclostomus (113 sp.), abundant in Antilles, also occurs in Madagascar, Arabia, Syria, Hungary and New Zealand; Tudora (34 sp.), Antilles, and one species in Algeria ; Cistula (40 sp.), Chondropoma (94 sp.), Bourcieria (2 sp.), Tropical America
Peculiar to or characteristic of the Palæarctic region are : Craspedopoma (5 sp.), confined to Madeira, the Azores and Canaries ; Leonia (1 sp.), Spain and Algeria ; Pomatias (22 sp.), Europe and Canaries with a species in the Himalayas ; Cecina (1 sp.), Manchuria.
The Ethiopian region has the peculiar genus Lithodion (5 sp.), Madagascar, Socotra and Arabia; and Otopoma (19 sp.), Mascarene Islands and Socotra, with a species in Western India and another in New Ireland.
The Australian region is characterised by Callia (3 sp.), in Ceram, Australia, and the Philippines respectively ; Realia (7 sp.), New Zealand and the Marquesas Islands; Omphalotropis (38 sp.), the Australian region, with some species in India, Malaya, and the Mauritius.
The remaining genus, Hydrocena (27 sp.), has a very widely scattered distribution, being found in South Europe, Japan, the Cape, China, Malaya, New Zealand, the Pacific Islands and Chili. From 10 to 20 per cent. of new species have been since described in most of the genera of this family.
FAMILY 30.—HELICINIDÆ, (7 Genera, 433 Species.) (1868.)
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