166 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



Family 5. OSTRACEA. 



Testa affixa, irregularis, foliata vel laminaris, raro auriculata, tenuis, 

 interdum translucida, valva una majore. Ligamentum internum 

 aut semi-internum. 



The Ostracea are somewhat allied to the Pectinacea, but differ in many 

 essential peculiarities ; and the passing out of a bony tendon through an 

 orifice in the shell in two of the genera is quite a new and particular 

 feature. Their shells also differ both in structure and composition ; the 

 valves are not radiated from the umbones like those of the Pectinacea, 

 nor are they so solid and calcareous, approaching rather, indeed, in com- 

 position to the fibro-laminar shells of the Aviculacea. Cuvier's family of 

 Les Ostrace'es was one of considerable extent, including both the families 

 of the Aviculacea and the Pectinacea ; but Les Ostrace'es of Lamarck con- 

 sist of five genera merely, three only of which are retained. We have 

 already noted in our observations on Vulsella (vide p. 149) the reason of 

 that genus being removed ; and the genus Gryphcea, which Lamarck also 

 included in this family, is united to Ostrea (vide p. 167), the incurva- 

 ture of the umbones, by which its shell is distinguished, being nothing 

 more than an inconstant variety of growth. We have, however, the 

 pleasure of introducing the new genus Placunanomia, which forms a sin- 

 gular and most important link in the distribution of this family. 



The shell of the Ostracea may be described as being attached, irregular, 

 either foliated or laminar, and rarely auriculated ; it is moreover thin, 

 sometimes quite translucid, and one valve is always larger than the 

 other. The ligament is either internal or semi-internal. 



The Ostracea are divided into four genera, as follows : 



Ostrea. Placunanomia. 



Placuna. Anomia. 



