EXTRALIMITAL LAND MOLLUSKS (GASTROPODA) FROM THE DEEP RIVER FORMATION, MONTANA - EVIDENCE FOR MESIC MEDIAL TERTIARY CLIMATE

Based on sedimentation patterns, lithology, and fossil vertebrates, th e period from late Eocene to late early Miocene has been considered a time of dry climate in the northern Rocky Mountains. Analysis of fossi l land snails from strata of Arikareean (late Oligocene to early Mioce ne) age from the Deep River Formation, Meagher County, Montana, howeve r, indicates a mesic climate with at least 75 cm/yr precipitation and suggests the presence of woodland with climatic requirements like thos e of present-day mixed mesophytic forest. Several species belong to ex tant genera whose modern range limits have been displaced eastward, of f the North American cordillera. Among these, two new species of land gastropod, Hendersonia stillmani (Helicinidae) and Euchemotrema (forme rly Stenotrema) occidaneum (Polygyridae), are described.