GENERA OF SHELLS. 41 



muscular impressions ; ligament external, tor- 

 tuous, somewhat penetrating within the shell. 



The etherise are very rare shells, and little known, 

 being attached to rocks in deep water. They resemble 

 ostrese in form, but are allied to the chamœ by their 

 muscular impressions, and are only distinguished from 

 them by having no tooth at the hinge. They are rather 

 lai'ge, and attached by the lower valve. 



(a.) An ohlong callosity in the base of the shell. 



E. elliptica, trigonula. 



(b.) Without the callosity. 



E. semilunata, transversa. 



CONCHIFERA UNIMUSCULOSA. 



Shell presenting internally one muscular impression, 



nearly in the centre. 

 Sect. I. — Ligament marginal, elongated on the 



MARGIN, SUBLINEAR. 



TRTDACNEA. 



Shell transverse, equivalve, miiscu'ar impression under the 

 '•middle of the superior margin, and produced to each side 

 under it. 



The shells of this family are regular, solid, and re- 

 markable by their sinuous or wavy superior margins. 



Tb-idacna, Shell regular, equivalve, ine- 

 quilateral, transverse, gaping at the lunule. 

 Hinge with two compressed, unequal, ad- 

 vanced and entering teeth ; ligament mar- 

 ginal, external. 



The tridacna is perfectly distinguished from hip- 

 popus by the lunule being always open and gaping, 

 through which the animal protrudes a byssus, by 



