TRILOBITES AT THE BASE OF THE MIDDLE ORDOVICIAN, WESTERN UNITED-STATES

Sections in the Basin Ranges provide stratigraphic standards for the O rdovician of the U.S., from which zones based on trilobites have been widely employed in regional correlation. This paper describes new tril obite faunas from a poorly known part of the succession, at the base o f the Middle Ordovician (Whiterockian) in western Utah and eastern Nev ada, and including the type Ibexian section near Ibex, Utah. The fauna s are from the topmost Wahwah, and overlying Juab Limestone Formations , in strata equivalent to Zone L. Although well-preserved, none of the trilobites are silicified. All are typical of inshore carbonate, Bath yurid biofacies. The majority are new species congeneric with, but dif ferent from species from the overlying Kanosh Formation. The dominant bathyurid is the hitherto little-known Psephosthenaspis, after which t he new Zone is named. A threefold subdivision of the Psephosthenaspis Zone is based on successive species of the genus. The lower two of the se are probably equivalent to the Valhallan Stage, described from more offshore biofacies from Spitsbergen. Eighteen species are described, of which nine are formally named as new including: Goniotelina ensifer , Petigurus inexpectatus, Psephosthenaspis microspinosa, M. glabrior, Pseudoolenoides aspinosus, Ectenonotus progenitor, Kanoshia reticulata and Pseudomera arachnopyge. The type species of a remarkable new illa enid-like bathyurelline, Madaraspis magnifica gen. et sp. nov., is des cribed.