145 



Oti on. Shell composed of two small, testaceous, semilunate, separate 

 valves, adhering near the sides of the aperture. 

 O. Cuvieri, O. Blainvillii. 



CONCHIFERA. 



Shell alivays bivalve, icholly 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; 

 19 



