Recognition of thecal modifications in the Mississippian blastoid Pentremites through geologic time has expanded its stratigraphic usefulness. Of particular stratigraphic significance are changes in the exterior shape, in the cross-sectional outline of the ambulacra, and in the number of hydrospire folds beneath each ambulacral side. Pentremites of Osage and early Meramec age have nearly flat bases, slightly convex ambulacra, and a low number of hydrospire folds per ambulacral side (normally three). In the late Meramec two divergent phylogenetic lines occur: Pentremites with flat or nearly flat bases and those with pyriform exterior shape. Both lineages persist to the end of the Mississippian. The lineage characterized by species with flat-based thecas have flat to slight y convex ambulacra from the late Meramec to the early Chester, and have progressive concavity of the ambulacra from the middle to late Chester. The number of hydrospire folds per ambulacral side in flat-based species changes from three in the late Meramec to four or five in the middle Chester; surprisingly, late Chester forms have a reduced number of folds (commonly three to four). The

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lineage with pyriform thecas have flat to convex ambulacra from the late Meramec to the early Chester; nearly flat or concave ambulacra occur in middle and late Chester specimens. The number of hydrospire folds per hydrospire group is four to five in early Chester pyriform species; five to seven in the middle Chester, and four to five in some late Chester forms.

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