Smith, Andrew B. Department of Paleontology, Natural History Museum, London, United Kingdom.
Last reviewed:January 2021
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An extinct order of irregular echinoids in the superorder Neognathostomata resembling clypeasteroids but lacking the accessory ambulacral pores characteristic of that group. Oligopygoids have well-developed petals, and there are characteristic small demiplates present below the petals. These demiplates are wedge-shaped and usually do not reach the inner surface of the test. Each has a simple ambulacral pore. The apical disk is monobasal and the mouth is oval and usually deeply sunken. Oligopygoids have a lantern, which closely resembles that of clypeasteroids, and their lantern muscle-attachment structures are a mixture of ambulacral and interambulacral processes.
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