1  definition  found 
 
  From  Webster's  Revised  Unabridged  Dictionary  (1913)  [web1913]: 
 
  Periostracum  \Per`i*os"tra*cum\,  n.;  pl  {Periostraca}.  [NL., 
  fr  Gr  ?  around  +  ?  shell  of  a  testacean.]  (Zo["o]l.) 
  A  chitinous  membrane  covering  the  exterior  of  many  shells;  -- 
  called  also  {epidermis}. 

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