NAUTILUS. Auct. (_A little boat._) _Fam._ Nautilacea, Lam. and Bl.--_Descr._ 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 modern authors, a thin shell, not chambered. The Nautili are known from the Ammonites by the septa being simple, not sinuated as in the latter genus, and in general the volutions 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 secondary strata, as low down as the Mountain limestone. N. pompilius, Frontispiece. NAYADES. Lam. A family of the order Conchifera Dimyaria, Lam. described as containing fresh-water bivalve shells, with or without teeth on the hinge. They are all pearly within, and have a thick, rather smooth epidermis without. This family contains a great variety of shells, which have been separated into an immense number of genera, but which G. B. Sowerby, sen. gives very good reasons for uniting under one generic name. The most generally received distinctions are as follows: 1. CASTALIA. Two cardinal, one lateral, ribbed teeth. This genus is removed from the family of Trigonacea. Fig. 140. 2. UNIO. Teeth various. Fig. 142, 145, 149, 148, 147, 151, 141. 3. HYRIA. Trigonal, alated. Fig. 143, 150. 4. ANODON. No teeth. Fig. 152. 5. IRIDINA. Hinge crenated. Fig. 150. NECTOPODA. Bl. The first family Nucleobranchiata, Bl. containing the genera Carinaria and Firola; the latter is not a shell. NEMATOPODA. Bl. The first class of the sub-type Malentozoa, Bl. containing all the mollusca with multivalve shells, except Chiton, and divided into the families Lepadicea and Balanidea, corresponding with Lamarck's sessile and pedunculated Cirripedes, and with the Linnæan genus Lepas. NEMATURA. Benson. _Fam._ Turbinacea, Lam.--_Descr._ Thin, nearly oval, somewhat compressed from back to front; spire acute, consisting of few rounded whorls; last whorl large, but contracted near the aperture; aperture small, oblique, rounded anteriorly; peritreme continuous, thin; operculum spiral, horny, with few volutions.--_Obs._ The distinguishing character of this genus is the contraction of the last whorl near the aperture, in which respect it is nearly resembled by the shell called Cyclostoma lucidum. Two recent and one fossil species, all very minute, are described by Sowerby in Loudon's Magazine of Natural History, New Series. Fig. 305.