396* CLASSIFICATION OF LAMELLIBRANCHIATA. 



C. 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. 



D. The MYTILACE^E, 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. 



E. 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. 



F. The ARCACE^E, or Arks, which have the mantle but 

 slightly adherent posteriorly, but adherent along the lower edge 

 of the shell : the foot is large ; and there are two adductor 

 muscles, of considerable size. 



II.- The SIPHONATA have the respiratory orifices distinctly 

 separated from each other, and usually produced into long 

 tubes. The lobes of the mantle are always more or less united. 

 These Mollusks may be arranged in four principal groups, 

 namely : 



A. The CHAHACE^E, or Clam-shells, in which the mantle is 

 closed, with a small aperture for the foot, which is very small ; 

 and the respiratory apertures also small and not produced into 

 siphons. The shell is usually very thick, and attached. 



B. The CARDIACEJE, or Cockles, which also have the mantle 

 closed, with an ample opening for the large and powerful foot ; 

 the siphons are usually short, and the pallial line entire or but 

 slightly sinuated behind. 



C. The VENERACE^:, with long separate siphons, and a dis- 

 tinctly sinuated pallial line. 



D. The PHOLADACE^E, with long siphons united throughoi 



