1908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 877 



recognized as members of the genus Xerita occur in the Upper 

 Cretaceous — the subgenus Otostoma, for instance, in the Upper 

 Cretaceous of Europe, Algeria, and Asia Minor. Making due 

 allowance for the fact that the distinction between Nerita and 

 Xenthia is not very obvious, especially in fossil shells, it is 

 clear that the numerous examples of the latter genus found in 

 freshwater tertiary deposits must have been derived from marine 

 forms that have long since disappeared from European seas, and 

 the existence of Neritina in European secondary strata pushes 

 back its origin to a remote period. It is probable on the pala?on- 

 tological evidence that the European species form a distinct 

 geographical group, and the coincidence of anatomical evidence 

 makes the probability well nigh a certainty. 



That Septaria is derived from Paranerita — the females of both 

 are triaulic — and that the latter is descended from the marine 

 Xerita still abundant in tropical seas, is beyond all doubt. But 

 the geographical distribution of these forms presents problems 

 Avhich become more difficult the more one reflects upon them. 

 The species of Paranerita that I have studied come from localities 

 as far apart as Fiji and Mauritiiis, but belong to the Indo-Pacific 

 region, the marine life of which is tolerably uniform in character. 

 The anatomy of these species is so similar that they are prac- 

 tically indistinguishable from one another, though I have noted 

 small differences between them. Paranerita, however, is only 

 exceptionally a marine form. Most of the species are iiuviatile, 

 some (e. g. P. cornea from the Philippines) are amphibious or 

 almost terrestrial in habit, and it is a singular thing that, 

 although largely continental, they abound in oceanic islands. 

 Similarly Septaria, an exclusively freshwater genus, is charac- 

 teristically insular, and species scarcely distinguishable from one 

 another are found in the Mascarene Islands, in Fiji, and other 

 Pacific islands. How have these freshwater forms reached their 

 present habitats ? Surely not by the ordinary means of dispersal, 

 for the animals, adapted as they are to existence in fresh Avater, 

 cannot have migrated over the whole Polynesian area, across 

 great extents of deep ocean. Nor could the egg-cases oi Septaria, 

 which are attached to the shell of the parent, have been waftecl 

 uninjured by any conceivable agency across the Indian Ocean. 

 If we fall back on the stereotyped explanation that the species 

 now isolated are representatives of a genus wliich is still widely 

 distributed and has been throughout long periods of geological 

 time, it is still insufficient, for it assumes what will not readily 

 be granted, the existence of former land-connections between 

 distant oceanic islands, between the Mauritius Islands, Samoa, 

 and Fiji. It is a tempting supposition that, as the marine 

 Xerita is xmiversally distributed in tropical seas and as Para- 

 nerita is abundant in rivers running into seas whei-e Xerita is 

 abundant, and as the anatomical characters of the two forms 

 are singularly alike, and as the conchological characters sepa- 

 rating Paranerita from Xerita are just those which are 



