. Bulletins of American paleontology. 22 Bulletin 331 ly middle Miocene, is considered to be a descendant of C clausa. The shell morphologies of the two species are nearly identical in ha%ing an umbilicus entirely closed by a semicircular umbilical callus (PI. 11). How- ever, C. ichishiana differs from C clausa in its oper- cular sculpture. The operculum of C ichishiana is smooth but may bear two weak striations along the outer margin (Majima, 1984). while C. clausa always has an entirely smooth operculum. Cryptonatica janthostoma is considered to be a de- scendant of C clausa. and first occur
. Bulletins of American paleontology. 22 Bulletin 331 ly middle Miocene, is considered to be a descendant of C clausa. The shell morphologies of the two species are nearly identical in ha%ing an umbilicus entirely closed by a semicircular umbilical callus (PI. 11). How- ever, C. ichishiana differs from C clausa in its oper- cular sculpture. The operculum of C ichishiana is smooth but may bear two weak striations along the outer margin (Majima, 1984). while C. clausa always has an entirely smooth operculum. Cryptonatica janthostoma is considered to be a de- scendant of C clausa. and first occurs in the lower middle Miocene Sankebetsu. Chikubetsu, and Taki- noue formations of central Hokkaido. Cryptonatica Janthostoma differs from C. clausa by having an um- bilicus that is commonly open. The degree of opening of the umbilicus in C. janthostoma shows ontogenetic variation (Text-figs. , ): juveniles (Text-fig. ) commonly have slightly open umbilici at their posterior portions where the sulcus is strongly incised (Text-fig. 13 [arrow a]), but a few juvenile spec- imens show entirely closed umbilici. The posterior portions of the umbilici of adults (Text-fig. ) are always weakly open (Text-fig. 13 [arrow b]). Gerontic specimens (Text-fig. ) have umbilici that are open along most of the umbilical callus margin, whereas their umbilici are ver>' shallow except for the posterior portions. Cryptonatica andoi is considered to be an offshoot oiC janthostoma and first occurs in the lower Pliocene Yonabaru Formation, Okinawa Prefecture, and in the lower Pliocene Kawabaru Member of the Koyu For- mation, Miyazaki Prefecture. It characteristically shows variation in the degree of umbilical opening (Text-figs. , ): juveniles (Text-fig. ) always show an entirely closed umbilicus but adults exhibit both closed (Text-fig. ) and open (Text- fig. ) umbilici. Adults with open umbilici are very similar to the gero
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