Thin-section examination of well-preserved specimens of the blastoid Globoblastus norwoodi from the Mississippian Burlington limestone of Iowa and Missouri has provided considerable new morphologic information. The anal deltoid, previously known to consist of an epideltoid and hypodeltoid, possesses 2 additional plates, the right and left cryptodeltoids. The number of hydrospire pores is not proportional to the number of side plates and brachioles as interpreted by early workers. The formation of the hydrospires from a single external spiracle at the calyx summit to their final disappearance at the aboral end of the ambulacrum is shown by a series of drawings based on detailed thin-section analysis of selected specimens.

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