Abstract
Pachytesta stewartii, a new species found in Middle Pennsylvanian coal ball deposits in southern Illinois, is 2.5 cm long and approximately 1.5 cm in width at its greatest diameter. Commissured ribs are pronounced near the apex and progressively less distinct toward the base. The integumentary system is delimited into 3 regions: outer sarcotesta, middle sclerotesta, and thin endotesta. Conspicuous in the new species is a pronounced basal extension with an enclosed chamber that may be the result of decomposition of thin-walled parenchyma cells. Relationships with other species and possible significance of the chalazal chamber are discussed.
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