  The second-layer phase diagrams of $^4$He and $^3$He adsorbed on graphite are
investigated. Intrinsically rounded specific-heat anomalies are observed at 1.4
and 0.9 K, respectively, over extended density regions in between the liquid
and incommensurate solid phases. They are identified to anomalies associated
with the Kosterlitz-Thouless-Halperin-Nelson-Young type two-dimensional
melting. The prospected low temperature phase (C2 phase) is a commensurate
phase or a $\textit{quantum hexatic}$ phase with quasi-bond-orientational
order, both containing $\textit{zero}$-$\textit{point}$ defectons. In either
case, this would be the first atomic realization of the $\textit{quantum liquid
crystal}$, a new state of matter. From the large enhancement of the melting
temperature over $^3$He, we propose to assign the observed anomaly of $^4$He-C2
phase at 1.4 K to the hypothetical supersolid or superhexatic transition.
