SILURIAN GASTROPODA FROM THE ALEXANDER TERRANE, PRINCE OF WALES ISLAND, SOUTHEAST ALASKA
The limited data base on Late Silurian gastropods from Alaska and Russia, indicates strong faunal ties between the gastropod faunas of the Alexander terrane with that of the Farewell terrane of southwestern Alaska and those described from the Ural Mountains by Chernyshev (1893). The most significant co-occurrence in southeast Alaska and the eastern Urals is Medfracaulus turriformis (Chernyshev, 1893). This distinctive species with its slow rate of expansion and deep umbilicus exhibits an unusual curved axis of coiling in some specimens. The curved spire is also seen in some shells of the same genus from west-central Alaska. Medfracaulus is characteristic of Upper Silurian strata of west-central Alaska (Nixon Fork subterrane of the Farewell terrane) and southeastern Alaska (Alexander terrane). Medfracaulus and similar morphotypes such as Coelocaulus karlae are unknown from rocks located undoubtedly on the North American continent (Laurentia) during the Late Silurian.
Other Heceta gastropods with Uralian affinities include Kirkospira glacialis which appears similar to Pleurotomaria lindströmi Oehlert of Chernyshev, 1893, and a species of Retispira that is similar to Bellerophon volgulicus Chernyshev, 1893. Beraunia is only known from the Silurian of Bohemia. Pachystrophia has previously been reported only from accreted terranes of western North American (Alaska and California) and Europe.