91 



Of all bivalve shells the single genus Trigo/iia, constituting a family of 

 itself, is the most remarkable, on account of the lustre and brilliant iri- 

 descence of its internal nacre. It has, too, of all bivalves, the most closely 

 interlocking hinge. The amateur prides himself on the display which the 

 pearly lining of the Trigonia valves, exquisitely rose or orange-tinted, makes 

 in his cabinet; and when separated, it requires no little ingenuity and 

 neatness of handling to close them again. The animal has a sharply an- 

 gled crenated-edged foot, and it would appear to possess considerable mus- 

 cular action; for Mr. Samuel Stutchbury, while collecting Trigonia in 

 Sydney Harbour, placed one on the gunwale of his boat, which leaped 

 overboard, clearing a ledge of four inches. They are confined to the Aus- 

 tralian Seas. 



Species. 



1. Lamarckii, Gray. 3. nobilis, Adams. 5. uniophora, Gray. 



2. margaritacea, Lam. 4. Strangei, id. 



Figure. 



Trigonia margaritacea. PI. 31. Fig. 177. Shell, with one valve 

 dropped to show its striately grooved hinge and rose-pearly interior. 



Family 4. ARCACEA. 



Shell ; generally equivalve, with the hinge composed with numerous 

 small teeth set either in a straight or a curved line. 



The shells of this family are characterized by a peculiarly distinct form 

 of hinge. The interlocking teeth are very numerous, fine, and irregular, 

 varying strikingly in detail in different species, and they are set either 

 comb-like in a straight line or in a curve. The ligament is external in 

 some genera, internal in others ; and the shell is in some forms of Arcacea 

 unusually stony and ponderous, while in others it has the lightness and 

 transparency of horn. The animals have the mantle-lobes freely open, 

 not formed into siphons, and they have mostly a deeply grooved byssus- 

 spinning foot. The genera are — 



Arca. CuculLjEa. Pectunculus. 



NUCULA. So-LENELLA. 



