. Bulletins of American paleontology. 48 Bulletin 331 on the umbilical u all. Umbilical wall is sculptured with incremental growth lines that are most distinct within the spiral groove. Anterior inner lip and basal lip mod- erately thickened. Discussion. —Polinices mizitnamiensis is character- ized by its ver\' small, elongate and perforate shell with two-and-one-half nuclear whorls. Because of its elon- gate shell, P. mizunamiensis is placed in Group C men- tioned in the discussion of the genus Polinices. It is well known that some species of Polinices with elongate shells (Group C) exhibit e
. Bulletins of American paleontology. 48 Bulletin 331 on the umbilical u all. Umbilical wall is sculptured with incremental growth lines that are most distinct within the spiral groove. Anterior inner lip and basal lip mod- erately thickened. Discussion. —Polinices mizitnamiensis is character- ized by its ver\' small, elongate and perforate shell with two-and-one-half nuclear whorls. Because of its elon- gate shell, P. mizunamiensis is placed in Group C men- tioned in the discussion of the genus Polinices. It is well known that some species of Polinices with elongate shells (Group C) exhibit entirely overlapping morphological variations. Marincovich (1977, pp. 248- 251) discussed the difficulty of dealing taxonomically with the Polinices iiber (Valenciennes, 1832) species- group, which includes P. iiher, P. iniemeratiis (Philippi, 1851), P. panamaensis (Recluz, 1844), and P. otis (Broderip and Sowerby, 1829), all living in the tropical eastern Pacific, and mentioned that "these species ex- hibit overlapping variations in form and umbilical ; This is also a problem in some modem western Pacific species of Polinices that are very similar to P. mizunamiensis. Kuroda and Kikuchi (1972) dis- cussed the taxonomic relations of three western Pacific species of Polinices, including Polinices tumidus (Swainson. 1840) [cited as (Tw/?;/5(Recluz, 1844), a junior s\non\m off. tumidus (Swainson, 1840), in Kuroda and Kikuchi (1972)], P. jlemingianus (Recluz, 1844), and P. mellosus (Hedley, 1924). The discussion of Kuroda and Kikuchi (1972), roughly translated from Japanese into English, follows: When Kikuchi traveled to Yaeyama, in the Ryukyu Islands, he col- lected a large number oi Polinices lumidus-hke specimens from Ka- bira Bay, Ishigaki-jima. Those specimens exhibit a wide range of continuous variations in their shell shapes, degree of umbilical open- ing, and coloration (from pure white to pale yellow). Based on shell morphologies, it seems imposs
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