A. 
MOLLUSCA. 825 
On this order, which many writers separate as a distinct class from the 
rest of the Molluscs, compare, 
Cuvier Mémoire sur Cephalopodes et sur leur anatomie, Mollusg., Mém. 1. 
with 4 plates. 
R. Owen Cephalopoda in Topp’s Cyclopedia, 1. pp. 517—562. 
Fiirussac et D’OrBIGNY Monographie des Cephalopodes cryptodibranches. 
Paris, 1834—1843. 18 livraisons. 
Tetrabranchiata. 
Family XII. Nawtilacea. Branchie four. Funnel cloven 
beneath, with two lamelle covering each other obliquely. Tenta- 
cles numerous, contractile, vaginate. Shell external revolute, spiral, 
polythalamous, hard; with septa perforated in the middle; last 
chamber ample, including the animal, with a membranous tube 
(siphon) produced from the posterior part of the abdominal sac 
through the foramina of the septa into the other chambers. 
Nautilus L. (in part), LAM. 
+ 
Sp. Nautilus Pompilius L., Rumpu. Amb. Rariteitk. Tab. xvii. figs. A, C, 
Bratnv. Malacol. Pl. tv. fig. 8; Guerin Iconogr., Mollusg. P. 1. fig. 7 ; 
the pearly nautilus; the shell attains a size of more than 3 foot; on the 
inside it is whitish with a pearly lustre, on the outside milk-white, with 
orange or red-brown stripes. This species, the only one of the seventeen 
species recorded by Linnavs that has remained in the genus Nautilus, is 
found in the Indian Ocean, the Islands of the Moluccan Archipelago, &e. 
Nautilus umbilicatus LaM., in the shell of which the last wreath does not 
cover that which precedes it, is in other respects comformable to the above. 
Notwithstanding the shell is not altogether rare, the animal of the pearly 
Nautilus was, until within the last few years, known only from a very im- 
perfect figure of RuMPHIUS. OWEN was the first who gave a good descrip- 
tion and a careful anatomy of this interesting animal (Memoir on the pearly 
Nautilus, with illustrations of its external form and internal structure. 
London, 1832, 4to). Besides the characters already given above, it is dis- 
tinguished from the rest of the Cephalopods now living by the absence of 
the Ink-sac. The head is covered by a large membranous hood, which 
represents exactly the circumference of the aperture of the shell, and arises 
from the expansion and coalescence of the cases of the uppermost pair of 
tentacles. The pediculated eye is partly covered by the over-hanging margin 
of the hood. Beside the 19 or 20 larger external tentacles, the mouth is 
surrounded by two pairs of flat, finger-shaped lappets, each of which encloses 
12—16 smaller tentacles, but in other respects similar in structure to the 
larger. If a male specimen observed by me be not a monstrosity, then it 
may be concluded that in these organs a sexual difference is presented ; 
(compare Tijdsschrift voor de Wis-en Natuurk. Wetenschappen, published by 
the Koninkl. Nederl. Instituut, 1. 1848, blz. 67—73, Pl. 1. figs. r—3), Trans. 
