The Mississippian blastoid PentremitesSay, 1820, has three different types of summit structures. Type 1 summits are characterized by long slender plates and are present in upper Meramecian and lower-upper Chesterian beds occurring from Iowa to Alabama. Type 2 summits consist of a massive pyramidal summit composed of large toothlike plates. They occur in lower and middle Chesterian deposits located from Illinois to Kentucky. Type 3 summit structures differ greatly from Types 1 and 2, generally consisting of six to seven rectangular plates arched over each side of individual spiracles. They have been found only in middle Chesterian sediments of Kentucky. In all three types of summit structures, the plates covering the anal spiracle are the most numerous and have a specialized arrangement of plates. Some anal spiracles are covered by alternating small spikelike plates, while others display a distinct anal sac or moundlike structure consisting of many small polygonal plates. Type 2 summits have many similarities with Type 1, and may have been derived from it. Type 3 summits, however, are radically different in type of plates and in their arrangement; their origin is not known. Type 3 summits are very common in the middle Chesterian.