STRATIGRAPHICALLY SIGNIFICANT EARLY AND MIDDLE ORDOVICIAN GASTROPOD OCCURRENCES, WESTERN AND NORTHWESTERN CANADA

Several biostratigraphically important gastropod genera are described from Alberta, British Columbia, Yukon Territory, and the District of M ackenzie. Malayaspira is present in the mid-Ibexian Kechika Group. Pal liseria, Maclurites, Teiichispira, Malayaspira, Barnesella, and Cerato pea occur in the Skoki Formation (highest Ibexian and lower Whiterocki an) and Helicotoma and Palliseria in the Sunblood Formation (lower Whi terockian). The largest known Teiichispira operculum is illustrated. T he forms are particularly useful for age determination within relative ly narrow limits and most of the taxa are readily identified in the fi eld.