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Blastoids were virtually unknown in post-Mississippian rocks of North America until about 50 years ago when abundantly occurring specimens of Pentremites were described from Morrowan beds in Oklahoma and Arkansas. The early Pennsylvanian age of these beds has been generally accepted by geologists for many years; some paleontologists, however, have questioned this age assignment because of the gross resemblance of type Morrow Pentremites to those of the upper Mississippian Chester. Morrowan Pentremites can be differentiated from those of the late Mississippian by a distinctive external shape and ambulacral cross-sectional outline and, internally, by the hydrospires which have a characteristic shape, thick walls, and a nearly constant number of hydrospire folds, except for reduced number of folds adjacent to the anus. In the field, latest Chester Pentremites commonly can be distinguished from those
of early Pennsylvanian age by the outline of the ambulacral cross section alone.
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