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Stylatractus pluto

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Amphisphaera, Stylatractus, and Xiphatractus
Richard N. Benson (04/11/2003)
Haeckel (1887) described the genera Amphisphaera, Stylatractus, and Xiphatractus as having three concentric lattice shells and two large radial spines in one axis. Differences are: (1) Amphisphaera (Haeckel, 1887, p. 142) has three spherical, not ellipsoidal, shells (although he admits [p. 121] that the distinction between ellipsoidal and spherical shells is sometimes difficult) and two free spines of equal size and similar form; (2) Stylatractus (p. 328) has three ellipsoidal lattice shells and two large opposite polar spines of equal size and similar form; and (3) Xiphatractus (p. 331) has a simple ellipsoidal cortical shell, double medullary shell, and two large polar spines of different size or form. Haeckel (1887) illustrated species of all three genera on Plate 17. In adhering to Haeckelís definitions, Benson (1966) assigned Gulf of California species that most closely resemble Haeckelís Plate 17 illustrations of Amphisphaera cronos and A. pluto to the genus Xiphatractus on the basis of their lattice shells being or tending to ellipsoidal and on slight difference in the size, not form, of the polar spines. Subsequently, Nigrini and Moore (1979) and Boltovskoy (1998) assigned species similar to if not the same as the Gulf of California species to Stylatractus. I now agree with their opinion that these three-shelled ellipsoidal species with two polar spines that are of similar form but not necessarily of equal size belong in Stylatractus. Species having polar spines of distinctly different form (Haeckel, 1887, Pl. 17, figs. 9-11) belong in Xiphatractus.



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