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. [Illustration: FIG. 17.--Two rows of the radula of _Cyclophorus_ sp., India, × 40.] 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] [Illustration: FIG. 18.--=A=, _Neritina reticularis_ Sowb., Calcutta (brackish water); =B=, _Helicina neritella_ Lam., Jamaica (land); =C=, _Proserpina_ (_Ceres_) _eolina_ Ducl., Central America (land).] =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.