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. _N. Pompilius._ The Pompilius Nautilus. Pl. 36, fig. 3. Species not umbilicated; the back rounded; aperture round and pearly; siphon sub-central; pale yellow, with chestnut streaks and undulations. _N. umbilicatus._ The umbilicated Pompilius. Pl. 36, fig. 1. Species umbilicated, sub-orbicular; pale fawn colour, with chestnut undated transverse clouds. FAMILY XXII. HETEROPODA. Two genera. 1. Argonauta. The Paper Sailor. Three species. The shells of this genus are remarkable for their fragility, delicacy, and elegance; they resemble a scroll, ornamented with various canaliculated grooves from the summit to the margin, which is bicarinated. The colour is usually bluish, but the keel is of a darker hue; they vary greatly in size. Shell navicular, symmetrical, very thin, compressed, bicarinated, sub-involuted longitudinally in the same plane; aperture very large, entire, symmetrical, square in front, slightly modified by the turn of the summit, and provided on each side with an earlike appendage, with thick and smooth edges; lips sharp. Argonauta argo. A. tuberculosa. A. nitida. _A. argo._ The Portuguese man-of-war. Pl. 35, fig. 1. Characterized above; shell whitish, fragile, keel rather narrow, with sharp-pointed tuberculations; sides striated transversely, wrinkled longitudinally. _A. tuberculosa._ The tuberculated Argonaut. Pl. 35, fig. 2. Species more convex at the sides, with nodulous elevations; keel broader, points more obtuse. 2. Carinaria. The glassy Nautilus. Three species.