200 NEMATURA. 



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, Fron- 

 tispiece. 

 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 con- 

 taining 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 Linnsean genus Lepas. 



NEMATURA. Benson. Fam. Turbinacea, Lam.— Bescr. Thin, 

 nearly oval, somewhat compressed from back to front - r spire 



