The evolution of the shell-sculpture in fresh-water snails of the family Viviparidæ

The peculiar sculptured shells of the Viviparidæ of Pliocene lake-basins in the old Austrian Empire and the Greek Archipelago have been discussed at length by several authors, notably in the first instance by Edward Forbes, and later by Brusina, by Tournouër, by Neumayr and by Newton. These authors have established the fact that in Slavonia and Dalmatia and in the island of Cos shells of the family ornamented with prominent spiral ridges gradually replaced those with a smooth surface in the course of the Pliocene age, while in the same countries the ridge shells have now disappeared and the Viviparidæ again have smooth shells. Neumayr has compared the Pliocene species with Nevill’s genusMargarya from Yunnan and (with greater justice in my opinion) withTulotoma Haldeman from North America. We possess details of the ancestry not only in the European and Levantine forms but also of that ofMargarya in Western China and of that of a very similar genus recently described by myself under the nameTaia from the Shan plateau of Upper Burma. Of these three series we have details of the living forms and of the types of environment they inhabit or inhabited only inTaia , fresh observations on which are given below. If we compare the majority of the shells of the European and Levantine ridge-shelled Viviparidæ with those of either of the two eastern genera a very close resemblance in the general character of the sculpture is at first sight the most salient feature, but a closer examination reveals (with other minor points which it is unnecessary to mention here) an almost constant difference in the number of ridges on each shell (cf . figs. 3, 4 and 6). In the Pliocene European forms there are rarely more than two spiral ridges on the body-whorl; whereas in sculptured shells of eitherTaia orMargarya there are almost always three main ridges and as a rule several subsidiary ridges, and one of the main ridges, that on the periphery of the whorl, is often much the most prominent. This does not seem an important difference if adult shells only are examined; but if we consider the soft parts of the living forms in correlation with the structure of the shell and compare the embryonic shell, it is seen to imply a constant anatomical difference.