NATICA. Brug. _Fam._ Neritacea, Lam. Hemicyclostomata, Bl.--_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. NATICARIA. Sw. A sub-genus of Natica, thus described: "Oval; convex above; umbilicus small, open, placed very near the top of the aperture; inner lip reflected; small. N. melanostoma, Mart. 189. f. 1926, 1927. cancellata, _Sw._ Ib. 189. f. 1939. bifasciata, Griff. Cuv. 1. f. 2." Sw. p. 346. NATICELLA. Guild. A sub-genus of Natica, thus described: "Operculum horny; shell globose, but generally depressed; umbilicus nearly filled up by a vitreous deposition of the inner lip; spire obtuse. N. aurantia. Mart. 189. f. 1934, 1935." Sw. p. 345. NAVICELLA. Lam. (_A little ship._) _Fam._ Neritacea, Lam. Hemicyclostomata, 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 whorl, and sometimes hiding the apex; apex incurved; operculum testaceous, flat, sub-quadrate, with a lateral articulation.--_Obs_. This well known genus, of which there are several species, is named Cimber by Montfort. The shells are brought from India, the Isle of France and the Moluccas. Fig. 323, N. elliptica. NAUTELLIPSITES. Parkinson. A generic name proposed to include such species of Nautilus as have been compressed, so as to assume an oval instead of a round form. The genus Ellipsolites of De Montfort consists of species of Ammonites similarly deformed. NAUTILACEA. Bl. The fifth family of Polythalamacea, Bl. the shells of which are described as more or less discoidal, compressed, symmetrically convolute; the last whorl much longer than the others; which are entirely hidden beneath it and advancing beyond the last but one, so as constantly to form a large oval aperture, which is always, however, modified by the last whorl. The septa are united in the greater number of instances and pierced by one or more (?) siphons. This family contains the genera Orbulites, Nautilus, Polystomella and Lenticulina. NAUTILACEA. Lam. The sixth family of Polythalamous Cephalopoda, Lam. containing the genera Discorbites, Siderolites, Polystomella, Vorticialis, Nummulites, Nautilus. To these may be added Simplegas and Endosiphonites. Fig. 472 to 476.