NEMATUEA. 213 



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. 

 Eive species are described in PI. ix. of the Author's Thesaurus 

 Conchyliorum. JS". pompilius, Prontispiece. 

 NATADES. 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 Gr. B. Sowerby, 

 sen. gives very good reasons for uniting under one generic name. 

 The most generally received distinctions are as foUows : 



1. Castalia. Two cardinal, one lateral, ribbed teeth. This 



genus is removed from the family of Trigonacea. Pig. 

 140. 



2. Unio. Teeth various. Pig. 142, 145, 149, 148, 147, 151, 



141. 



3. Htkia. Trigonal, alated. Pig. 143, 150. 



4. Anodon. No teeth. Pig. 152. 



5. Ibidika. Hinge crenated. Pig. 150. 



NEAEA. Grray, 1830. A genus composed of bivalve shells, 

 formerly included in the genus Anatina, having a small spoon- 

 shaped process and posterior lateral tooth in one valve, and an 

 undefined ligamental pit, with no lateral tooth in the other. Ex. 

 JN". longirostratum, fig. 493, 4 ; and another species, fig. 495. 



NECTOPODA. Bl. The first famUy Nucleobranchiata, Bl. con- 

 taining the genera Carinaria and Pirola ; the latter is not a 

 sheU. 



NEMATOPODA. Bl. The first class of the sub-type Malen- 

 tozoa, Bl. containing aU the mollusca with multivalve shells, 

 except Chiton, and divided into the families Lapadicea and 



. Balanidea, corresponding with Lamarck's sessile and peduncu- 

 lated Cirripedes, and with the Linnean genus Lepas. 



NEMATTJEA. Benson. Fam. Tiorbinacea, Lam. — Descr. 

 Thin, nearly oval, somewhat compressed from back to front ; 



