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Living pleurotomariids or slit shells provide a rare glimpse at the biology of remnants of a primitive and predominantly Paleozoic group of gastropods. Discovery of the first living pleurotomariid (Fischer and Bernardi 1856) generated considerable scientific excitement, in precisely the same pattern that later attended the discovery of Neopilina galatheae (Lemche 1957).

The living fossil mollusc examples used here represent three different phylogenetic patterns. The neopilinid monoplacophorans have a disjunct time pattern that separates the living species from the Cambro-Devonian tryblidiid (limpet) monoplacophorans—a gap just short of 430 million years—with the extinction of the last tryblidiid Archaeophiala in the Lower Devonian. The Silurian Pilina unguis (Lindstrom 1880) from the middle Silurian of Gotland is nearly identical (except for shell thickness) to Vema (Vema) ewingi Clarke and Menzies 1959 (Fig. 1).