. Bulletins of American paleontology. Japanese Cenozoic Naticids: Majima 21 (PI. 7, figs. 17, 21-24), whereas its earliest known rep- resentatives in early Pleistocene faunas have a greatly depressed spire (PI. 7, figs. 18-20). The latter individ- uals are very similar to the depressed variant of G. didyma dainichiensis (PI. 7. figs. 13-15), which is con- sidered herein to be a forerunner off/, vcsicalis. Gloss- aulax didyma dainichiensis is, therefore, considered to be a transitional population between G. didyma (in- cluding G. didyma colicazae and G. didyma didyma) and G. vesicalis. It shows
. Bulletins of American paleontology. Japanese Cenozoic Naticids: Majima 21 (PI. 7, figs. 17, 21-24), whereas its earliest known rep- resentatives in early Pleistocene faunas have a greatly depressed spire (PI. 7, figs. 18-20). The latter individ- uals are very similar to the depressed variant of G. didyma dainichiensis (PI. 7. figs. 13-15), which is con- sidered herein to be a forerunner off/, vcsicalis. Gloss- aulax didyma dainichiensis is, therefore, considered to be a transitional population between G. didyma (in- cluding G. didyma colicazae and G. didyma didyma) and G. vesicalis. It shows substantial morphological overlap with both G. didyma and G. vcsicalis. The inferred speciation process of G. vesicalis is con- formable to that documented by Williamson (1981). Williamson studied late Cenozoic lacustrine molluscan faunas of the Turkana Basin in northern Kenya and concluded that these faunas provide the first fine-scaled palaeontological resolution of events during speciation: fundamental phenotypic transformation of both sexual and asexual taxa occur rapidly, in comparatively large populations, and is accompanied by a significant elevation of phe- notypic variance. This increase in variance reflects extreme devel- opmental instability in the transitional populations. Glossaulax didyma dainichiensis, a transitional pop- ulation between G. didyma and G. vesicalis. shows a significant elevation of phenotypic variance confirmed by its wide range of morphological variation. Lineage III In the Euspira melsensis (Makiyama, 1926) - E. marincovichu n. sp. - E. mitsuganoensis Shibata, 1970 lineage, E. melsensis first occurred in Oligocene faunas, whereas both E. mahncovlchi and E. mitsuganoensis first appeared in early middle Miocene time. The mor- phological range of E. mahncovlchi occupies an inter- mediate position between those of the other two species and serves to link them. The base of E. mahncovlchi ranges from rounded (Text-fig. ) to angulate (Text- fig. 1
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