In dentistry, calculus or tartar refers to hardened plaque on the teeth, formed by aging in the presence of saliva, debris[?], and minerals. Its rough surface provides an ideal medium for bacterial growth, threatening the health of the gums[?] and absorbing unaesthetic stains far more easily than natural teeth.
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... by the narrow and shallow Strait of Tartary or Mamiya Strait, which often freezes in winter in its narrower part, and from Yezo (Japan) by the Strait of La Pérouse. ...