Figure 11. Grzybowskia jasoni. Oblique median section through a microspheric specimen. The protoconch is particularly small, at about 15 ±3 µm, but may not be through the centre of the sphere, as indicated by the relatively thick (obliquely sectioned?) enclosing wall. There are about four whorls without subdivison of the chambers [juvenile stage]. This series is followed by a single whorl of the adult stage, with minimal subdivision of the chambers before the onset of the gerontic stage, seen on the left edge here, where the whorl height increases and multiple, apparently irregular, chamberlets occur.

Thin section FFC 1C is the maximum dimension of the largest (central) test is 2.74 mm. Whole rock thin section from yellow limestone, Flying Fish Cove, Christmas Island.