A new species of Late Ordovician callocystitid rhombiferan (Glyptocystitidae), Lepadocystis decorus, is described from the Brainard Shale of north-central Illinois. This species is characterized by five pectinirhombs, open infralateral and lateral plate circlets, and well-developed ridges on the thecal plates. Lepadocystis decorus well illustrates that open vs. closed plate circlets in glyptocystitids can largely be a function of pectinirhomb length and is therefore in part an ontogenetically linked characteristic. Consequently, great care must be taken to assure maturity when using this feature in taxonomy.
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