Snails

In general, this name is given to the land molluscs of the family Helicidae, or even to others, including freshwater species, whose shells resemble those of that family.

The natural history of the Madeiran pulmonates has been carefully investigated by various malacologists, such as Richard Thomas Lowe, the Baron of Castelo de Paiva and others, but no two authors agree on the number of species recorded. If one holds that certain forms deserve the rank of species, another asserts that they are no more than mere varieties, and sometimes the reverse view is taken of other forms.

If we accept the opinion of Thomas Vernon Wollaston, the most recent of the authors to have dealt with the subject at length, there are 176 species, of which 131, counting only the land species, are peculiar to these islands.

Our archipelago is therefore a centre of origin for many species of pulmonates. What makes this part of the fauna still more remarkable is that, of these 131 species, 61 are peculiar to Madeira, 44 to Porto Santo and 10 to the Desertas, while only 16 are common to all the islands. In this count the Desertas are taken as a whole; if we consider them as what they really are, namely three islands, it follows that only 5 species can be regarded as belonging to all the islands of the archipelago. It is clear, then, that each island has in turn given rise to forms entirely its own, and that distribution has taken place over areas so limited that even small islets, isolated rocky outcrops in the sea, have seen forms arise among their rocks that have never been found outside so restricted a habitat.

Thus the islets of Cal, Fora and Fonte, off Porto Santo, have forms of Helix and Pupa found nowhere else. In other cases the habitat is confined to the crests of neighbouring hills or to the slope of a ravine, as happens both on Porto Santo and on Madeira. The genus Helix has the greatest number of representatives, followed by Pupa, Lovea, Patula and

Clausilia, with successively fewer species, besides other genera of lesser importance. The slugs are divided among 10 species, and 4 species are recorded in the operculate genus Craspedopoma, one of which, C. trochoideum, has been found only in certain valleys in the north of Madeira. The freshwater species include the genera Limnaea, Physa, Planorbis and Ancylus, all of whose members are European; they are common in the levadas (irrigation channels), streams and tanks, where an operculate of the order of prosobranchs, Hydrobia similis, also occurs. On the coastal rocks washed by the tide, endemic species of the genera Melampus and Auricula have been collected, as well as other African or European species of the genera Pedipes, Truncatella and Assiminea.

Many of the Madeiran pulmonates have been observed in the sub-fossil state in prodigious quantities, their dense aggregation sometimes giving them the appearance of conglomerates. The deposits of these fossils consist either of calcareous sand, sometimes compacted as if it were sandstone, or of clay mixed with angular stones; in the latter case they are clastic rocks or transported ground, almost always formed on the flanks of certain hills. Such deposits exist on Madeira, east of Caniçal (‘reed bed’); on the summit of Bugio, one of the Desertas; and on Porto Santo at various places, such as Ilhéu de Baixo, Ilhote, Vigia da Ponta, Campo de Baixo, Ribeiro da Vigia, Penedo, Mornos and Zimbral da Areia.

Of the 176 species of pulmonates – a figure established according to Thomas Vernon Wollaston’s criteria, as already stated – 82 have been found in the fossil state, and of these only 12 have not been observed living, so they may be considered extinct. Helix Lowei, peculiar to Porto Santo and the largest of all the snails of the archipelago, measuring 5 centimetres or more in diameter, was long considered extinct. However, the botanist J. M. Moniz succeeded in discovering a living specimen on the rocks of Ilhéu de Cima (‘upper islet’), and the English naturalist Dr Alfred Merle Norman obtained from a shell dealer in Frankfurt a recent, well-preserved specimen which had evidently left these islands by unknown means and ended up in the German dealer’s shop.