Land operculates attain a most extraordinary development in the Greater Antilles, and constitute, in some cases, nearly one-half of the whole Molluscan fauna. Several groups of the Cyclostomatidae find their headquarters here, and some spread no farther. On the mainland this prominence does not continue. West Indian influence is felt in Central America and on the northern coast district, and some Antillean genera make their way as far as Ecuador. The whole group entirely disappears in Chili and Argentina, becoming scarce even in Brazil.

Among the fresh-water operculates, Ampullaria is abundant, and widely distributed. Vivipara, so characteristic of N. America, is entirely absent. Chilina, a remarkable fresh-water pulmonate, akin to Limnaea, is peculiar to Chili, Patagonia, and Southern Brazil, but is not found in the tropical portion of the continent. Of the fresh-water Pelecypoda Mycetopus, Hyria, Castalia, Leila, and Mülleria are peculiar forms, akin to the Unionidae.

(1) The Antillean Sub-region surpasses all other districts in the world in respect of (1) extraordinary abundance of species, (2) sharp definition of limits as a whole, (3) extreme localisation of the fauna of the separate islands. The sub-region includes the whole of the half-circle of islands from the Bahamas to Grenada, together with the extreme southern end of the peninsula of Florida, which was once, no doubt, a number of small islands like the Bahamas. Trinidad, and probably Tobago, although containing an Antillean element, belong to the mainland of S. America, from which they are only separated by very shallow water.

The sub-region appears to fall into four provinces:—

(a) Cuba, the Bahamas, and S. Florida; (b) Jamaica; (c) San Domingo (Haiti), Porto Rico, and the Virgin Is., with the Anguilla and St. Bartholomew group; (d) the islands from Guadeloupe to Grenada. The first three provinces contain the mass of the characteristic Antillean fauna, the primary feature being the extraordinary development of the land operculates, which here reaches a point unsurpassed in any other quarter of the globe. The relative numbers are as follows:—

It appears, then, that the proportion of operculate to inoperculate species, while very high in Cuba (about 41 per cent of the whole), reaches its maximum in Jamaica (where the operculates are actually in a majority), begins to decline in San Domingo (about 40 per cent), and continues to do so in Porto Rico, where they are not more than 24 per cent of the whole. These operculates almost all belong to the families Cyclostomatidae and Helicinidae, only two genera (Aperostoma and Megalomastoma) belonging to the Cyclophorus group. Comparatively few genera are absolutely peculiar to the islands, one or two species of most of them occurring in Central or S. America, but of the several hundreds of operculate species which occur on the islands, not two score are common to the mainland.

The next special feature of the sub-region is a remarkable development of peculiar sub-genera of Helix. In this respect the Antilles present a striking contrast to both Central and S. America, where the prime feature of the land Pulmonata is the profusion of Bulimus and Bulimulus, and Helix is relatively obscured. No less than 14 sub-genera of Helix, some of which contain species of almost unique beauty and size, are quite peculiar to the Greater Antilles, and some are peculiar to individual islands.

Here, too, is the metropolis of Cylindrella (of which there are 130 species in Cuba alone), a genus which just reaches S. America, and has a few species along the eastern sea-board of the Gulf of Mexico. Macroceramus and Strophia are quite peculiar; the former, a genus allied to Cylindrella, which attains its maximum in Cuba and San Domingo, is scarcely represented in Jamaica, and disappears south of Anguilla; the latter, a singular form, resembling a large Pupa in shape, which also attains its maximum in Cuba, is entirely wanting in Jamaica, and has its last representative in S. Croix. One species irregularly occurs at Curaçao.

The carnivorous group of land Mollusca are represented by several peculiar forms of Glandina, which attain their maximum in Jamaica and Cuba, but entirely disappear in the Lesser Antilles.

A certain number of the characteristic N. American genera are found in the Antillean Sub-region, indicating a former connexion, more or less intimate, between the W. Indies and the mainland. The genera are all of small size. The characteristic N. American Hyalinia are represented in Cuba, San Domingo, and Porto Rico; among the Helicidae, Polygyra reaches Cuba, but no farther, and Strobila Jamaica. The fresh-water Pulmonata are of a N. American type, as far as the Greater Antilles are concerned, but the occurrence of Gundlachia (Tasmania and Trinidad only) in Cuba is an unexplained problem at present. Unionidae significantly occur only at the two ends of the chain of islands, not reaching farther than Cuba (Unio 3 sp.) at one end, and Trinidad (which is S. American) at the other.

A small amount of S. American influence is perceptible throughout the Antilles, chiefly in the occurrence of a few species of Bulimulus and Simpulopsis. The S. American element may have strayed into the sub-region by three distinct routes: (1) by way of Trinidad, Tobago, and the islands northward; (2) by a north-easterly extension of Honduras towards Jamaica, forming a series of islands of which the Rosalind and Pedro banks are perhaps the remains; (3) by a similar approximation of the peninsula of Yucatan and the western extremity of Cuba. Central America is essentially S. American in its fauna, and the characteristic genera of Antillean operculates which occur on its eastern coasts are sufficient evidence of the previous existence of a land connexion more or less intimate (see map).

(a) Cuba is by far the richest of the Antilles in land Mollusca, but it must be remembered that it is also much better explored than San Domingo, the only island likely to rival it in point of numbers. It contains in all 658 species, of which 620 are land and 38 fresh-water, the land operculates alone amounting to 252.

Carnivorous genera form but a small proportion of the whole. There are 18 Glandina (which belong to the sections Varicella and Boltenia) and 4 Streptostyla, the occurrence of this latter genus being peculiar to Cuba and Haiti (1 sp.) among the Antilles, and associating them closely with the mainland of Central America, where Streptostyla is abundant. These two genera alone represent the Agnatha throughout the sub-region.

There are no less than 84 species of Helix, belonging to 12 sub-genera. Only one of these (Polymita) is quite peculiar to Cuba, but of 7 known species of Jeanerettia and 8 of Coryda, 6 and 7 respectively are Cuban. Thelidomus has 15 species (Jamaica 3, Porto Rico 3); Polydontes has 3, the only other being from Porto Rico; Hemitrochus has 12 (Jamaica 1, Bahamas 6); Cysticopsis 9 (Jamaica 6); Eurycampta 4 (Bahamas 1).

The Cylindrellidae find their maximum development in Cuba. As many as 34 Macroceramus occur (two-thirds of the known species), and 130 Cylindrella, some of the latter being most remarkable in form (see Fig. 151, B, p. 247).

The land operculates belong principally to the families Cyclostomatidae and Helicinidae. Of the former, Cuba is the metropolis of Ctenopoma and Chondropoma, the former of which includes 30 Cuban species, as compared with 1 from San Domingo and 2 from Jamaica. Megalomastoma (Cyclophoridae) is also Haitian and Porto Rican, but not Jamaican. Blaesospira, Xenopoma, and Diplopoma are peculiar. The Helicinidae consist mainly of Helicina proper (58 sp.), which here attains by far its finest development in point of size and beauty, and of Eutrochatella (21 sp.), which is peculiar to the three great islands (Jamaica 6 sp., San Domingo 6 sp.).

The Bahamas, consisting in all of more than 700 islands, are very imperfectly known, but appear to be related partly to Cuba, partly to San Domingo, from each of which they are separated by a narrow channel of very deep water. They are certainly not rich in the characteristic groups of the Greater Antilles. The principal forms of Helix are Plagioptycha (6 sp.), common with San Domingo, and Hemitrochus (6 sp.), common with Cuba. Strophia is exceedingly abundant, but Cylindrella, Macroceramus, and Glandina have but few species. There are a few species of Ctenopoma, Chondropoma, and Cistula, while a single Schasicheila (absent from the rest of the sub-region) forms a link with Mexico.

Southern Florida, with one or two species each of Hemitrochus, Cylindrella, Macroceramus, Strophia, Ctenopoma, and Chondropoma, belongs to this province.

(b) Jamaica.—The land Mollusca of Jamaica are, in point of numbers and variety, quite unequalled in the world. There are in all as many as 56 genera and more than 440 species, the latter being nearly all peculiar. The principal features are the Glandinae, the Helicidae, and the land operculates. The Glandinae belong principally to the sub-genera Varicella, Melia, and Volutaxis, Streptostyla being absent, although occurring in Cuba and San Domingo. There are 10 genera of Helix, of which Pleurodonta is quite peculiar, while Sagda (13 sp.) is common only with S.W. San Domingo (2 sp.), and Leptoloma (8 sp.) only with Cuba (1 sp.). The single Strobila seems to be a straggler from a N. American source. Macroceramus has only 2 species as against 34 in Cuba, and of Cylindrella, in which Cuba (130 sp.) is so rich, only 36 species occur. The genus Leia, however (14 sp.), is all but peculiar, occurring elsewhere only in the neighbouring angle of San Domingo, which is so closely allied with Jamaica. The complete absence of Strophia is remarkable.

The land operculates form the bulk of the land fauna, there being actually 242 species, as against 221 of land Pulmonata, a proportion never again approached in any part of the world. As many as 80 of these belong to the curious little genus Stoastoma, which is all but peculiar to the island, one species having been found in San Domingo, and one in Porto Rico. Geomelania and Chittya, two singular inland forms akin to Truncatella, are quite peculiar. Alcadia reaches its maximum of 14 species, as against 4 species in San Domingo and 9 species in Cuba, and Lucidella is common to San Domingo only; but, if Stoastoma be omitted, the Helicinidae generally are not represented by so many or by so striking forms as in Cuba, which has 90 species, as against Jamaica 44, and San Domingo 35.

(c) San Domingo, although not characterised by the extraordinary richness of Cuba and Jamaica, possesses many specially remarkable forms of land Mollusca, to which a thorough exploration, when circumstances permit, will no doubt make important additions. From its geographical position, impinging as it does on all the islands of the Greater Antilles, it would be expected that the fauna of San Domingo would not exhibit equal signs of isolation, but would appear to be influenced by them severally. This is exactly what occurs, and San Domingo is consequently, although very rich in peculiar species, not equally so in peculiar genera. The south-west district shows distinct relations with Jamaica, the Jamaican genera Leia, Stoastoma, Lucidella, and the Thaumasia section of Cylindrella occurring here only. The north and north-west districts are related to Cuba, while the central district, consisting of the long band of mountainous country which traverses the island, contains the more characteristic Haitian forms.

The Helicidae are the most noteworthy of the San Domingo land Mollusca. The group Eurycratera, which contains some of the finest existing land snails, is quite peculiar, while Parthena, Cepolis, Plagioptycha, and Caracolus here reach their maximum. The Cylindrellidae are very abundant, but no section is peculiar. Land operculates do not bear quite the same proportion to the Pulmonata as in Cuba and Jamaica, but they are well represented (100 to 152); Rolleia is the only peculiar genus.

The relations of San Domingo to the neighbouring islands are considerably obscured by the fact that they are well known, while San Domingo is comparatively little explored. To this may perhaps be due the curious fact that there are actually more species common to Cuba and Porto Rico (26) than to Porto Rico and San Domingo. Cuba shares with San Domingo its small-sized Caracolus and also Liguus, but the great Eurycratera, Parthena, and Plagioptycha are wholly wanting in Cuba. The land operculates are partly related to Cuba, partly to Jamaica, thus Choanopoma, Ctenopoma, Cistula, Tudora, and many others, are represented on all these islands, while the Jamaican Stoastoma occurs on San Domingo and Porto Rico, but not on Cuba, and Lucidella is common to San Domingo and Jamaica alone. An especial link between Jamaica and San Domingo is the occurrence in the south-west district of the latter island of Sagda (2 sp.). The relative numbers of the genera Strophia, Macroceramus, and Helicina, as given below (p. 351), are of interest in this connexion.

Porto Rico, with Vièque, is practically a fragment of San Domingo. The points of close relationship are the occurrence of Caracolus, Cepolis, and Parthena among the Helicidae, and of Simpulopsis, Pseudobalea, and Stoastoma. Cylindrella and Macroceramus are but poorly represented, but Strophia still occurs. The land operculates (see the Table) show equal signs of removal from the headquarters of development. Megalomastoma, however, has some striking forms. The appearance of a single Clausilia, whose nearest relations are in the northern Andes, is very remarkable. Gaeotis, which is allied to Peltella (Ecuador only), is peculiar.

Land Mollusca of the Greater Antilles

The Virgin Is., with St. Croix, Anguilla, and the St. Bartholomew group (all of which are non-volcanic islands), are related to Porto Rico, while Guadeloupe and all the islands to the south, up to Grenada (all of which are volcanic), show marked traces of S. American influence. St. Kitt’s, Antigua, and Montserrat may be regarded as intermediate between the two groups. St. Thomas, St. John, and Tortola have each one Plagioptycha and one Thelidomus, while St. Croix has two sub-fossil Caracolus which are now living in Porto Rico, together with one Plagioptycha and one Thelidomus (sub-fossil). The gradual disappearance of some of the characteristic greater Antillean forms, and the appearance of S. American forms in the Lesser Antilles, is shown by the following table:—

(d) In Guadeloupe we find Cyclophorus, Amphibulimus, Homalonyx, and Pellicula, which are characteristic of S. America, and nearly all recur in Dominica and Martinique. These islands are the metropolis of Dentellaria, a group of Helix, evidently related to some of the forms developed in the Greater Antilles. Stragglers occur as far north as St. Kitt’s and Antigua, and there are several on the mainland as far south as Cayenne. Traces of the great Bulimus, so characteristic of South America, occur as far north as S. Lucia, where also is found a Parthena (San Domingo and Porto Rico). Trinidad is markedly S. American; 55 species in all are known, of which 22 are peculiar, 28 are common to S. America (8 of these reach no farther north along the islands), and only 5 are common to the Antilles, but not to S. America. The occurrence of Gundlachia in Trinidad has already been mentioned.

The Bermudas show no very marked relationship either to the N. American or to the West Indian fauna. In common with the former they possess a Polygyra, with the latter (introduced species being excluded) one species each of Hyalosagda, Subulina, Vaginula, and Helicina, so that, on the whole, they may be called West Indian. The only peculiar group is Poecilozonites, a rather large and depressed shell of the Hyalinia type.

(2) The Central American Sub-region may be regarded as extending from the political boundary of Mexico in the north to the isthmus of Panama in the south. It thus impinges on three important districts—the N. American, West Indian, and S. American; and it appears, as we should perhaps expect, that the two latter of these regions have considerably more influence upon its fauna than the former. Of the N. American Helicidae, Polygyra is abundant in Mexico only, and two species of Strobila reach N. Guatemala, while the Californian Arionta occurs in Mexico. S. American Helicidae, in the sub-genera Solaropsis and Labyrinthus, occur no farther north than Costa Rica. Not a single representative of any of the characteristic West Indian Helicidae occurs. Bulimulus and Otostomus, which form so large a proportion of the Mollusca of Venezuela, Colombia, Ecuador, and Peru, together with Orthalicus, are abundant all over the region. Again, Cylindrella, Macroceramus, and some of the characteristic Antillean operculates, are represented, their occurrence being in most cases limited to the eastern coast-line and eastern slope of the central range.

Besides these external elements, the region is rich in indigenous genera. Central America is remarkable for an immense number of large carnivorous Mollusca possessing shells. There are 49 species of Glandina, the bulk of which occur in eastern and southern Mexico; 36 of Streptostyla (S.E. Mexico and Guatemala, only 1 species reaching Venezuela and another Peru); 5 of Salasiella, 2 of Petenia, and 1 of Strebelia; the last three genera being peculiar. Streptaxis, fairly common in S. America, does not occur. Velifera and Cryptostracon, two remarkable slug-like forms, each with a single species, are peculiar to Costa Rica. Among the especial peculiarities of the region are the giant forms belonging to the Cylindrellidae, which are known as Holospira, Eucalodium, and Coelocentrum (Fig. 232). They are almost entirely peculiar to Mexico, only 7 out of a total of 33 reaching south of that district, and only 1 not occurring in it at all.

The land operculates are but scanty. Tomocyclus and Amphicyclotus are peculiar, and Schasicheila, a form of Helicina, occurs elsewhere only in the Bahamas. Ceres (see Fig. 18, C, p. 21) and Proserpinella, two remarkable forms of non-operculate Helicinidae (compare the Chinese Heudeia), are quite peculiar. Pachychilus, one of the characteristic fresh-water genera, belongs to the S. American (Melaniidae) type, not to the N. American (Pleuroceridae). Among the fresh-water Pulmonata, the Aplecta are remarkable for their great size and beauty. In the accompanying table “Mexico” is to be taken as including the region from the United States border up to and including the isthmus of Tehuantepec, and “Central America” as the whole region south of that point.

Land Mollusca of Central America

(3) The Colombian Sub-region includes Colombia, New Grenada, Venezuela, Guiana, Ecuador, Peru, and Bolivia. It has been usual to separate off the two latter countries as forming a distinct “Peruvian” sub-region; but there is, as will be seen, absolutely no line to be drawn between the Mollusca of Peru and those of Ecuador; nor would one, on geographical considerations, expect to be able to draw such a line. A better method of subdivision, so far as the species of the whole eastern portion of the region are concerned, would be to group the Mollusca according to the altitude at which they occur, were it not that the evidence on this point is at present but fragmentary. We know, however, that all along the line of the Andes certain species, more particularly of Bulimulus, occupy their own zones of elevation, some ascending as high as 10,000 feet above the sea, and never occurring on the plains.

In the northern portions of this sub-region, Central American and West Indian influence is felt to a certain extent. Thus there are eight Glandina and one Streptostyla in Venezuela and Colombia together with one or two species of Cistula, Chondropoma, Proserpina, and Cylindrella, while a single Strophia (decidedly a straggler) occurs at Curaçao. In Demerara and Cayenne there are three or four species of Dentellaria. In Ecuador, however, Glandina diminishes to three species, and in Peru disappears altogether, although one Streptostyla occurs. Similarly the West Indian operculates are reduced to one Chondropoma (Ecuador) and disappear entirely in Peru.