396 CLASSIFICATION OF LAMELLIBRANCHIATA. 



but the branchial lamina? are not adherent ; there is but a single 

 adductor muscle ; and there is usually the rudiment of a foot. 



III. The MARGARITACE^, or Pearl-Oyster tribe; these, also, 

 have the edges of the two halves of the mantle, and also of the 

 branchial laminae, free, or not adherent to each other ; and there 

 is but a single adductor muscle ; but the foot is larger, and 

 a byssus is usually present, by which the animals attach them- 

 selves to rocks, &c. 



IV. The MYTILACKE, or Mussels, which have the lobes of the 

 mantle adherent posteriorly; the foot small, and provided with 

 a byssus ; and a double adductor muscle, of which the anterior is 

 small. 



Y. The UNIONID.E, or Fresh-water Mussels, which have the 

 mantle non-adherent, but which have the branchial laminae 

 united posteriorly, so as to form a sort of siphon for the exit of 

 water. The foot is a large fleshy mass ; and there is no byssus. 



In the preceding Orders, the hinge of the shell was very simple ; 

 it now becomes of increased complexity. In all the remaining 

 Orders, the adductor muscle is double. 



VI. The ARC AGILE, or Arks, which have the mantle but 

 slightly adherent posteriorly, but adherent along the lower edge 

 of the shell : the foot is large ; and both adductor muscles are of 

 considerable size. 



VII. The CAMACE^E, or Clamp-shells, have the mantle still 

 more adherent, but always divided by a large opening at the 

 lower part for the passage of the foot, which is of great size. The 

 respiratory orifices are not prolonged into tubes, but are sur- 

 rounded by a circle of radiating tentacula. 



VIII. The CARDIACE^E, or Cockle tribe, which have the lobes 

 of the mantle still partly open in front, but prolonged into tubes 

 at the posterior extremity ; these Mollusks have a large foot, 

 which they use for burying themselves in the mud. 



IX. SOLENID^J, or Solens, and their allies, in which the mantle 

 is quite closed in front, so as only to allow the foot to pass out, 

 and is prolonged behind into tubes of considerable length ; which 

 can, however, be withdrawn into the shell. 



X. The INCLUSA, in which these characters are still further 



