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 ORDER II. CONCHIFERA UNIMUSCULOSA. 



Shell presenting internally, one muscular impression, nearly in the centre. 

 This order is divided into three sections. 



SECTION I. Ligament marginal, elongated on the margin, sublinear. 

 This section contains three families, — Tridacnea, Mytilacea, and Mallacea. 



TRIDACNEA.— Two Genera. 



Shell transverse, equivalve, the muscular impression under the middle of 

 the superior margin, and is prolonged to each side under it. 



Tridacna. Shell regular, equivalve, inequilateral, transverse, gaping at 

 the lunula; hinge with two compressed, unequal, anterior, entering teeth. 

 Ligament marginal, external. 



T. gigas, T. squamosa, T. mutica, 1 fossil species. 



' elongata, ' crocea, ' serrifera, 



Hippopus. Shell equivalve, inequilateral, transverse; the lunula close; 

 hinge with two compressed, unequal, anterior, and entering teeth. Ligament 

 marginal, external. 



H. maculatus. 



MYTILACEA.— Two Genera. 



The ligament at the hinge subinternal, marginal, linear, very entire, occu- 

 pying a great part of the anterior margin; rarely foliated. 



Modiola. Shell subtransverse, equivalve, regular, the posterior side very 

 short; beaks nearly lateral, depressed on the short side; hinge without teeth, 

 lateral, linear. Ligament marginal, subinternal; one elongated, club-shaped, 

 sublateral, muscular impression. 



