An involute, symmetrical, discoid shell, whose whorls do not touch each
other; the septa or partitions are brilliant pearl, concave externally,
pierced by a tube called the siphon or siphuncle, placed close to the
inner edge of the aperture; covered with a thin epidermis.
_S. Peronii._ Peron’s Spirula. Pl. 36, fig. 2.
Answers to the above description; colour yellowish white.