Diastems are common features in the Plattin limestone (Middle Ordovician, Black River) in Missouri. Two prominent examples found by the writer in eastern Jefferson County are described. One is shown by the discordant molds of two large orthocone cephalopods, the second, more characteristic, by an intraformational conglomerate. Convergence of the Plattin from Cape Girardeau to St. Louis County, Missouri is due to a reduction of the number of members and thinning of the members. Part of the thinning is attributed to frequent depositional hiatuses shown by diastems. The occurrence of closely spaced diastems in the Middle Ordovician in Central Pennsylvania is cited.