MOLLUSCA. 505 



stalk arising from the abdomen. The margin of the cloak 

 is surrounded with two rows of tentacula, some of which, 

 in the external row, have greenish tuberculated summits. 

 The mouth is surrounded with numerous branched tenta- 

 cula, in place of the four ordinary labial appendages. 



B. Ostreadte. Shell cemented to foreign bodies. Body 

 destitute of a foot. 



To this family the following genera are related : Ostrea, 

 Spondylus, and Anomia. The last genus ought to form a 

 family apart. It is distinguished by the singular character 

 of the adductor muscle, a portion of which is attached to 

 the corneous or testaceous plate, which passes through the 

 cardinal perforation, and adheres to rocks. There is a 

 small foot, which is capable of being likewise protruded 

 through the cardinal perforation. In the genus SPONDY- 

 LUS, the margin of the cloak is fringed with a double row 

 of tentacula having tuberculated summits, and the foot is 

 seated on a short stalk, with a large radiated disk. 



2d Tribe. 



Shell closed by two adductor muscles. 



The two genera, Avicula and Meleagrina (of LAMARCK), 

 form one family of this tribe ; the genus Pinna another ; 

 and the Arcadae a third, including Area, Pectunculus, Nu- 

 cula, Cucullsea Trigonia, and Castalia. 



2d Subdivision. 



Cloak more or less closed, forming syphons. 



The further division of this group depends on the modi- 

 fications of the syphons, or aperture of the cloak. 



1st Tribe. 



The union of the cloak forms only one syphon. 



This syphon is situate posteriorly opposite the anus, and 

 serves for the issuing of the excrements. The other large 

 opening allows the water to enter to the mouth and gills. 



