  The masses of tetraquark states of all $q\bar q c\bar c$ and $c\bar c c\bar
c$ quark configurations are evaluated in a constituent quark model, where the
Cornell-like potential and one-gluon exchange spin-spin coupling are employed.
All model parameters are predetermined by comparing the theoretical and
experimental masses of light, charmed and bottom mesons. The theoretical
predictions of the charmonium-like tetraquarks are compared with the observed
$XYZ$ states, and one tentative assignment is suggested. The work suggests that
the $X(6900)$ observed by LHCb is likely to be the first radial excited
fully-charm tetraquark state with $J^{PC} = 0^{++}$ in the $\bar 3_c \otimes
3_c$ configuration, and the ground and second radial excited states of
fully-charm tetraquark are around $6450MeV$ and $7250MeV$ respectively.
