1. Spirula. One species.
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.
2. Nautilus. Two species.
An elegant, well-known shell, more or less ventricose, discoid, slightly compressed, umbilicated or not, but never papillose; the septa simple, transverse, not visible externally, the last deeply sunk and perforated by a siphon running through them all; edges entire.
The N. Pompilius, when dissected, displays its beautiful pearly chambers; fine specimens are often converted into drinking-cups by the Orientals, who sometimes remove the outer coating, so that its whole appearance is pearly.
The Nautilis varies in size; some are microscopic; and although they have received different names, and on account of the animal have been made to form different genera, it was deemed unnecessary to treat of them here.
Nautilus Pompilius.
Nautilus umbilicatus.