NAYADES. 69: 
NA’TICA. Lam. Fam. Neritacea, Lam. Hemicyclostomata, BI. 
Descr. Globose, thick, generally smooth; spire short, pointed, 
with few volutions; aperture semilunar, entire; outer lip thin; 
columellar lip oblique, nearly straight, callous; umbilicus with a 
spiral callosity, terminating behind the columella, and sometimes 
filling up the cavity; operculum shelly in some species, horny in 
others ; epidermis thin, light, semitransparent. Obs. The straight, 
callous, smooth edge of the columella, and the callosity serve to 
distinguish this genus from Nerita, Neritina, Neritopsis and 
Helix. Fig. 327, 328. 
NANINIA. Gray. A genus composed of the planorbicular species 
of Helix, with large umbilici, included in the sub-genus Heli- 
cella, Fer. Hz. H. citrina, fig. 280. 
NAVICEL'LA. Lam. (A little ship.) Fam. Neritacea, Lam. He- 
micyclostomata, Bl. Descr. Transversely oval, symmetrical, 
smooth; aperture entire, oval; dorsal surface convex; outer lip 
thin; inner lip flat, straight edged, spread over the front surface 
of the body whorls, and sometimes hiding the apex; apex in- 
curved; operculum testaceous, flat, sub-quadrate, with a lateral 
articulation. Obs. This wéll known genus, of which there are 
several species, is named Cimber by Montf. The shells are 
brought from India, the Isle of France and the Molluccas. Fig. 
323, N. elliptica. 
NAUTELLIP’SITES. Parkinson. Exuiesitites, Montf. 
NAUTILA’CEA. Bl. The fifth family of Polythalamacea, BI. 
containing Orbulites, Nautilus, Polystomella and Lenticulina. 
NAUTILA’CEA. Lam. The sixth family of Polythalamous 
Cephalopoda, Lam. containing the genera Discorbites, Sidero- 
lites, Polystomella, Vorticialis, Nummulites, Nautilus. To these 
may be added Simplegas and Endosiphonites. 
NAU’TILUS. (A little boat.) Fam. Nautilacea, Lam. and BI. 
Deser. Convolute, discoid, chambered, symmetrical; spire partly 
or entirely concealed by the last whorl; aperture modified by 
the last whorl, wide, sinuated on the dorsal margin; interior 
surface pearly; septa dividing the chambers simple; siphon 
discontinuous. Obs. The shell named Nautilus by Pliny, is the 
Argonauta of later authors, a thin shell, not chambered. The 
Nautili are known from the Ammonites by the septa being sim- 
ple, not sinuated as in the latter genus, and in general the volu- 
tions of the spire are not visible. ‘Three or four species are 
known inhabitants of the Pacific Ocean, and Australian Ocean. 
The fossil species are found in the tertiary, and also in the se- 
condary strata, as low down as Mountain Limestone. Fig. 474, 
N. pompilius. 
NA’YADES. Lam. A family of the order Conchifera Dimyaria, 
Lam. containing the genera Unio, Hyria, Anodonta, and Iridina, 
with other genera enumerated in explanation of figures, 141 
to 152. 
