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placoid scale

A Dictionary of Zoology | 1999 | | © A Dictionary of Zoology 1999, originally published by Oxford University Press 1999. (Hide copyright information) Copyright

placoid scale (dermal centicle) A type of scale that comprises the basic unit of the hard skin cover of sharks. It consists of a hard base embedded in the skin and a spiny process (cusp); these are covered by vitrodentine. The scale projects outwards and backwards and it is the cusps that are responsible for the roughness of the shark's skin. The bulk of the scale consists of dentine surrounding a central pulp cavity. Unlike the scales of bony fish, placoid scales stop growing after they reach a certain size and new scales are added as the animal grows.

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