Abstract

The following middle Carboniferous gastropods are described from the western U.S.: Lunulazona costata n. gen. & n. sp., Lunulazona sp. A. Glabrocingulum binodosodum n. sp, Glabrocingulum quadrigatum, n. sp., Glabrocingulum n. sp. aff. G. inflatum, and Mourlonia sp. Preliminary collecting indicates that the Chainman Formation contains 3 gastropod zones, as follows in ascending order: Lunulazona costata zone, Glabrocingulum quadrigatum zone, and the Glabrocingulum binodosum zone. The gastropod zones are widely recognized fossil zones. The new genus Lunulazona seems to be restricted to the striate Goniatites zone of Visean age. Glabrocingulum quadrigatum is common in the early Namurian Eumorphoceras zone. Glabrocingulum binodosum occurs in the lower half of the Rhipidomella nevadensis zone also of Namurian age. The ontogenetic development of surficial ornamentation and growth is presented by described quantitative techniques and graphs. Only 2 ontogenetic stages are distinctly recognized, protoconch and post protoconch.

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