FRUSTULAR MORPHOLOGY OF SOME CENTRIC DIATOM SPECIES FROM MIOCENE FRESH-WATER SEDIMENTARY-ROCKS OF WESTERN USA AND CANADA

The morphology of Actinocyclus cedarensis Lohm. ex Brad. et Krebs and A. tubulosus Churs. from the middle Yakima Basalt (middle Miocene) of Grant County, Washington, an A. theleus Brad. et Krebs from early Mioc ene (23.5 m.y.) tuffaceous sedimentary rocks of Churchill County, Neva da, compared favorably with the morphology of these three species from other previously described localities. A middle Miocene diatomite (11 -12 m.y.) from the Esmeralda Formation of Esmeralda County, Nevada, yi elded Thalassiosira nevadica, n. sp. Observations on the morphology of Aulacoseira (Melosira) canadensis Hust., A. praeislandica Jouse, A. g ranulata (Ehr.) Ralfs, A. praedistans Jouse, and related taxa revealed that these taxa are often identifiable only with great difficulty.