ABSTRACT
Examination of nearly 500 specimens of Mitrocystites mitra, and Mitrocystella incipiens from the Lower Ordovician of Bohemia housed in the Barrande and Hanus collections of the National Museum, Prague, Czech Republic and new material from the Horǎk Museum, Rokycany, has yielded a wealth of new morphological data. Peripores and lateripores in M.mitra are surrounded by fine-mesh stereom clearly indicating they are sutural pores in origin. Internal tubular structures associated with these pores are probably gonadal in function; not eyes (pores) and nervous system as viewed in Jefferies’ ‘concept Calcichordata’. Both M.mitra and M. incipiens have a unique fully plated cloaca associate with the digit-like anal valvular plates. Mitrocystella incipiens, in some specimens, retains vestigial lateripores and the newly discovered aulacophore is longer than the theca. Within the Mitrocystitidae we can observe the possession and loss of basic echinoderm structures i.e. external hydropore and sutural pores. In addition, stereom construction of the plates, sutural pore systems, and well developed anal valves are thecal characters shared with the Blastozoa.
