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ANATINIDiE. 



This tribe consists of a very natural assemblage of ace- 

 phalous mollusks, closely allied to those of the last two 

 families. The shells it includes are of a more or less ob- 

 long form, often tumid, sometimes compressed, mostly fra- 

 gile in comparison with their dimensions ; all inequivalve 

 and gaping at the hinder extremity. The surface is dull- 

 coloured or white, rarely conspicuously ornamented, though 

 often minutely scabrous. The hinge varies, being in some 

 furnished with spoon-shaped fulcra, in others deprived of 

 such substitutes for teeth, and furnished with a free shelly 

 ossicle fixed to each valve by a cartilage. The ligament is 

 internal, or both internal and external. The animals have 

 mantles, closed, except where the foot, which is more or less 

 linguiform, protrudes. Their siphons are separated, mode- 

 rately long, and furnished with fringed orifices. They are 

 closely allied to the members of the last family in the cha- 

 racters of the respiratory system, a single branchial leaflet 

 only being developed fully on each side. They live buried 

 in mud or sand, sometimes in cavities of rock, and have a 

 wide range, extending from low water mark to very gi'eat 

 depths. Members of this family are distinctly recognized 

 as fossil during the early stages of the oolitic period. 



