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varying the specific gravity of the 

 shell; so that it sinks when the 

 pericardial fluid is forced into the 

 siphuncle, and becomes buoyant, 

 when the same fluid returns to the 

 pericardium. The substance of the 

 siphuncle is a thin and strong 

 membrane, surrounded by a coat of 

 muscular fibres, by which it could 

 contract or expand itself, in the 

 process of admitting or ejecting 

 any fluid to or from its interior. 

 When the arms and body are ex- 

 panded, the fluid remains in the 

 pericardium, and the siphuncle is 

 empty, and collapsed, and sur- 

 rounded by the portions of air 

 that are permanently confined 

 within each chamber ; in this state 

 the specific gravity of the body and 

 shell together is such as to cause 

 the animal to rise, and be sustained 

 floating at the surface. When, on 

 any alarm, the arms and body are 

 contracted, and withdrawn into the 

 shell, the retraction of these parts, 

 causing pressure on the pericar- 

 dium, forces its fluid contents into 

 the siphuncle, and as the quantity 

 of matter within the shell is thus 

 increased, without any increase of 

 magnitude, the specific gravity of 

 the entire animal is increased, and 

 it begins to sink. Rumphius states 

 that, at the bottom of the sea, the 

 nautilus creeps with his boat above 

 him. 



Fossil remains of the nautilus 

 are found in strata from the moun- 

 tain limestone upwards. In some 

 of these the siphunculus is beauti- 

 fully preserved. But while, as a 

 genus, the nautilus occurs in for- 

 mations of every age, from the 

 transition series upwards, yet cer- 

 tain species appear limited to par- 

 ticular geological formations. The 

 eocene, miocene, and pliocene has 

 each its particular nautili. Buck- 

 land) Kirby. Owen. Parkinson. 

 Sowerly. 

 NAUTILUS SY'FHO. The name given 



to a very beautiful, camerated, 

 siphuncled fossil shell, found in the 

 tertiary strata at Dax, near Bour- 

 deaux. This fossil presents devia- 

 tions from the usual characters of 

 the nautilus, whereby it approxi- 

 mates to the ammonite. 



IsTAu'TiLtrs zic ZAC. A fossil, camera- 

 ted, siphuncled shell, found in the 

 London clay. This and the naut- 

 ilus sypho appear to form connect- 

 ing links between the genera Naut- 

 ilus and Ammonite. 



NECRO'PHAGOUS. (from veKpo?, dead, 

 and (pa^etv, to eat, Gr.) Animals 

 which devour dead substances. 

 The unclean animals, with respect 

 to their habits and food, were 

 divided into two classes ; namely, 

 zoophagous animals, or those which 

 attack and devour living animals; 

 and necrophagous animals, or those 

 which devour dead ones, or any 

 other putrescent substance. 



NE'CROMITE. (from veKpo?, dead, Gr.) 

 A mineral occurring in small mas- 

 ses in limestone ; found near Balti- 

 more. When struck, it exhales a 

 fetid odour, resembling that of 

 putrid flesh ; from this quality it 

 obtained its name. 



NE'CTARY. That part of a flower 

 which secretes and contains the 

 honey, (an almost universal fluid 

 in flowers) and is either a part of 

 the corolla, or an organ distinct 

 from it, and variously formed, as in 

 the monks'-hood, black hellebore, 

 &c. ; or it is a tubular elongation 

 of the calyx, or of a petal ; or, an 

 assemblage of glands. 



NEE'DLE ORE. r lhe Nadelerz of Wer- 

 ner. Colour steel-gray. Amor- 

 phous, or in acicular hexaedral 

 prisms, which are occasionally in- 

 vested with a yellowish or greenish 

 crust. Fracture uneven and me- 

 tallic. Specific gravity 6*8. Con- 

 stituents, bismuth 43-6, lead 24-50, 

 copper 12-1 2, sulphur 11 '60, nickel 

 1-58, tellurium 1'32. 



NEMATOIDE'A. The fourth order of 



