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Otion. Shell composed of two small, testaceous, semilunate, separate 

 valves, adhering near the sides of the aperture. 



O. Cuvieri, O. Blainvillii. 



conchifp:ra. 



Shell always bivalve, wholly or partly covering the animal; sometimes free, 

 sometimes fixed ; the valves mostly joined at the margin by a hinge or ligament. 

 The shell is sometimes enlarged by testaceous, accessary pieces, not belonging 

 to the shell. 



This class is divided into two orders, viz. — Conchifera Bimusculosa and 

 Conchifera Unimusculosa. 



ORDER I. CONCHIFERA BIMUSCULOSA. 



The shell presents, in the interior, two separate and lateral muscular im- 

 pressions. This order is divided into four sections, viz. — C. Crassipeda, 

 C. Tenuipeda, C. Lamellipeda, and C. Ambigua, or the Chamacea. 



SECTION I. CONCHIFERA CRASSIPEDA. Shell gaping at the sides 



when shut. 



This section contains four families, — Tubicolaria, Pholadaria, Solenacea, 

 and Myaria. 



TUBICOLARIA.— Six Genera. 



Aspergillum. Sheath tubular, testaceous, gradually attenuating to the 

 anterior end, which is open; the other extremity larger, and club-shaped; 



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