MOLLUSCA. 825 



On this order, which many writers separate as a distinct class from the 

 rest of the Molluscs, compare, 



CUVIEE Mtmoire sur Cephalopodes et sur leur anatomic. Mollusq.,M^m. i. 

 with 4 plates. 



E. OWEN Cephalopoda in TODD'S Cyclopedia, I. pp. 517562. 



FERUSSAC et D'ORBIGNY Monographic des Cephalopodes cryptodibranches. 

 Paris, 1834 1843. 18 livraisons. 



A. TetroJbrancJiiata. 



Family XII. Nautilacea. Branchiae four. Funnel cloven 

 beneath, with two lamellae 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., EUMPH. Amb. Rariteiik. Tab. xvu. figs. A, c, 

 BLAINV. Malacol. PI. iv. fig. 8 ; GUERIN Iconogr., Molhisq. P. I. fig. 7 ; 

 the pearly nautilus; the shell attains a size of more than ^ 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 LINNAEUS that has remained in the genus Nautilus, is 

 found in the Indian Ocean, the Islands of the Moluccan Archipelago, &c. 

 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 EUMPHIUS. 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 CephaJopods 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 1 6 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 vow de Wis-en NatuurTc. Wetenschappen, published by 

 the KoninJd. Nederl. Instituut, r. 1848, biz. 6773, PI. I. figs. 13), Trans. 



