Mooi, Rich Department of Invertebrate Zoology and Geology, California Academy of Sciences, San Francisco, California.
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The name originally given by G. Cuvier in 1817 (later formalized by P. A. Latreille in 1825) to what is now regarded as an unnatural (nonmonophyletic, that is, not evolved from a common ancestor) group of echinoids (sea urchins, sand dollars, and allied forms). Following Latreille, influential classification schemes, including one determined by J. W. Durham and R. V. Melville in 1957, divided sea urchins into two cognate groups: the “regular” urchins, in which the anus was at the summit of the body (test), directly opposite the mouth; and the “irregular” urchins, in which the anus was moved in the direction of what is now recognized as a posterior interambulacrum (number 5 in Lovén's numbering system; the interambulacrum is the area between two ambulacra, which are the radial series of plates along which the tube feet are arranged). Ironically, it was suggested that the irregular urchins were derived from several independent stocks, making it an unnatural group, whereas the Regularia was a valid taxon. However, it is now accepted that the Irregularia is monophyletic, as suggested by A. B. Smith and phylogenetically demonstrated by T. Saucède and colleagues, who showed that irregular echinoids are derived from Lower Jurassic forms and should be grouped formally under the Irregularia. In fact, it is the “Regularia” that is an unnatural group. The recognition of Irregularia implies arbitrary exclusion from the proposed “Regularia.” A recent analysis and reclassification by A. Kroh and A. B. Smith is in agreement with this view.
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