A new rhipidocystid echinoderm from the Upper Ordovician Chambersburg Formation in western Maryland (USA) is here described based on four exquisitely preserved specimens. Specimens of Durhamicystis americana gen. et sp. nov. preserve both thecal sides and the oral area, with identification of major apertures including peristome, periproct, gonopore, and hydropore. This allows orientation of this species and proper comparison across all rhipidocystids. Durhamicystis americana has a large theca composed of ten marginal plates with only two basals on the posterior side, two ambulacra and seven large oral plates with brachioles attached either on orals or smaller flooring plates. Rhipidocystids include taxa with lateral flattening along the BC-DE axis (i.e., Rhipidocystis) and others along the anterior-posterior A-CD axis (i.e., Durhamicystis, Neorhipidocystis, and Petalocystites) suggesting rhipidocystids may be paraphyletic.

Key words: Echinodermata, Blastozoa, Rhipidocystidae, Paleozoic, Chambersburg Formation, North America.

Samuel Zamora [s.zamora@igme.es], Instituto Geológico y Minero de España (IGME), C/Manuel Lasala, 44, 9ºB, 50006, Zaragoza, Spain and Grupo Aragosaurus-IUCA, Área de Paleontología, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza, Spain. James Sprinkle [echino@jsg.utexas.edu], Department of Geological Sciences, Jackson School of Geosciences, University of Texas, 1 University Station C1100, Austin TX, 78712-0254, USA. Colin D. Sumrall [csumrall@utk.edu], Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996-1410, USA.