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Color is variable but is usually slaty black to bluish black above and paler below.
Only two teeth become well developed, one on each side of the lower jaw, and these may be lost in old age. In most species they are exposed when the mouth is closed and are used in intraspecific combat. In females the functional teeth are much smaller than those of males and often do not erupt above the gums. There may be small nonfunctional teeth in both jaws. A row of such vestigial teeth on each side of the upper jaw seems to be characteristic of M. grayi.