About 133 species of blastoids have been assigned to the genus Pentremites, of which 63 are here recognized as valid species or varieties; 30 are synonyms or unrecognizable, 10 are nomina nuda, and 30 belong to other genera. They range from Lower Mississippian to Late Mississippian, possibly to Lower Pennsylvanian, reaching their acme in number and variety in the lower and middle Chester. Their structures are defined and evaluated, and the valid species are redescribed and figured. The evolution of the genus and species are attempted, and the ranges and phylogenies of the species given. All species are analyzed and described after a standard plan, and keys are formulated, so that the species can be more readily and reliably identified and geologic horizons thereby determined. Three new species are proposed: Pentremites gutschicki, Pentremites halli, and Pentremites malotti.