The power of the Littorinidae to live for days or even weeks without being moistened by the sea may be verified by the most casual observer. In the tropics this power seems even greater than on our own shores. I have seen, in various parts of Jamaica, Littorina muricata living at the top of low cliffs among grass and herbage. At Panama I have taken three large species of Littorina (varia, fasciata, pulchra), on trees at and above high-water mark. Cases have been recorded in which a number of L. muricata, collected and put aside, have lived for three months, and L. irrorata for four months.[29] These facts are significant, when we know that the land operculates almost certainly originated in a tropical climate.

The Cyclophoridae, Cyclostomatidae, and Aciculidae, which, as contrasted with the other land operculates, form one group, have very close relations, particularly in the length and formation of the radula, or lingual ribbon, with the Littorinidae.

On the other hand, the Helicinidae, Hydrocenidae, and Proserpinidae are equally closely related to Neritina. The Proserpinidae (restricted to the Greater Antilles, Central America and Venezuela) may perhaps be regarded as the ultimate term of the series. They have lost the characteristic operculum, which in their case is replaced by a number of folds or lamellae in the interior of the shell. It has already been noticed how one group of Neritina (Neritodryas) occurs normally out of the water. This group furnishes a link between the fresh-water and land forms. It is interesting to notice that here we have the most perfect sequence of derivatives; Nerita in the main a purely marine form, with certain species occurring also in brackish water; Neritina in the main fresh-water, but some species occurring on the muddy shore, others on dry land; Helicina the developed land form; and finally Proserpina, an aberrant derivative which has lost the operculum.[30]

Gasteropoda.—(2) Pulmonata. The origin of these, the bulk of the land fauna, must at present be regarded as a problem not yet finally solved. Some authorities, as we have seen, regard them as derived from the Nudibranchiate, others, probably more correctly, from the Tectibranchiate Opisthobranchs.

The first known members of the land Pulmonata (Pupa [?], Hyalinia) are from the Carboniferous of North America. Similar but new forms appear in the Cretaceous, from which time to the present we have an unbroken series. The characteristically modern forms, according to Simroth,[31] are Helices with thick shells. According to the same author, Vitrina and Hyalinia are ancestral types, which give origin not only to many modern genera with shells, but to many shell-less genera also, e.g. Testacella is probably derived through Daudebardia from Hyalinia, while from Vitrina came Limax and Amalia. A consideration of the radulae of the genera concerned certainly tends in favour of these views.

Godwin-Austen, speaking generally, considers[32] genera of land Pulmonata with strongly developed mantle-lobes and rudimentary shell as more advanced in development than genera in which the shell is large and covers all or nearly all the animal.