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Baculites was an uncoiled ammonite. Fragments of its shells, like this one, are very common in the Pierre Shale. Occasionally, a nearly complete shell is found. The various species are distinguished by their suture patterns, cross-sectional shapes, degree of taper, size, and surface ornament. The book by N. F. Larson et al., which is cited on the main Pierre Shale page, has a key for distinguishing between 24 species of Baculites found in the Pierre Shale. This specimen seems to belong to the unnamed species which is described in the book by N. F. Larson et al. on p. 26. The suture pattern is similar to that of Baculites scotti. The prominent ribs distinguish it from Baculites scotti. (Length 12 cm.)
A more complete specimen of a Baculites from the "Worldwide Museum of Natural History," still lacking part of the narrow end, can be seen here.
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