New species of Baculites from the Upper Cretaceous of Montana and South Dakota

Baculites mariasensis, B. sweetgrassensis and B. gregaryensis are described as new. The first two are of Niobrara age, and the last is from rocks of Judith River age. The baculite zones of the northern Great Plains are reviewed. M ANY collections of fossils from the Upper Cretaceous rocks of the western interior United States contain several undescribed species of baculites that are of value in correlation. Two new species, Baculites mariasensis and B. sweetgrassensis, occur in shale and sandstone equivalents of the Fort Hays limestone member of the Niobrara formation. B. gregoryensis, n. sp. , occurs in marine rocks equivalent to the Judith River formation of central Montana. The recognition of these species makes it possible to construct a more complete Upper Cretaceous time scale for the Western Interior based on Baculites. The zones constitute the following sequence in the northern Great Plains. [B. grandis B. compressus Montana age B. gregoryensis B. asperiformis B. aquilaensis B. thomi B. codyensis Colorado age B. mariasensis B. cf. besairiei (B. gracilis Baculites gracilis Shumard characterizes the Greenhorn limestone of the Black Hills 1 Published by permission of the Director, U. S. Geological Survey. and the equivalent calcareous beds in the middle of the Colorado shale of northcentral Montana. Baculites cf. besairiei Collignon occurs in the Carlile shale of the lack Hills and in the equivalent part of the Colorado shale of Montana. B. mariasensis, n. sp., is present in the upper part of the Colorado shale of Montana and in the upper part of the Frontier formation of westc ntral Wyoming. B. codyensis Reeside, of upper Niobrara age, is abundant in the Cody shale of Wyoming and in the upper part of the Colorado shale of Montana (above the zone of B. mariasensis). B. thomi Reeside occurs in the uppermost part of the Colorado shale of north-central Montana and in the Telegraph Creek formation of north-central and south-central Montana. B. aquilaensis Reeside characterizes the Eagle sandstone of Montana, the Gammon ferruginous member of the Pierre shale of the Black Hills, and parts of the Steele and Cody shales of Wyoming. B. asperiformis Meek is the most common form in the Claggett shale of Montana and in the Mitten black shale member of the Pierre shale of the Black Hills. B. gregoryensis, n. sp. , occurs in the calcareous Gregory member of the Pierre shale in the Missouri Valley of South Dakota, and in a 200-foot sandy shale member of the Pierre EXPLANATION OF PLATE 118 All figures natural size. FIGS. 1-5-Baculites gregoryensis Cobban, n. sp. 1, Lateral view of holotype; 2-5, lateral views of four paratypes. Gregory member of Pierre shale, 2 miles west of Oacoma, Lyman County, South Dakota. (p. 820) 6-9-Baculites sweetgrassensis Cobban, n. sp. 6, 7, Lateral and ventral views of paratype; 8, 9, lateral and ventral views of holotype. Colorado shale, 5.5 miles south of Shelby, Toole County, Montana. (p. 820) 10-12-Baculites mariasensis Cobban, n. sp. 10, 11, Lateral views of paratypes; 12, lateral view of holotype. Colorado shale, 5.5 miles south of Shelby, Toole County, Montana. (p. 818)