Specific heat and neutron diffraction measurements have been made to study in detail the commensurate-incommensurate (C-IC) transition of ortho-deuterium monolayers physisorbed on graphite. Several novel intermediate phases between the commensurate and the incommensurate phase have been observed, which we denote as α, β, γ, δ and ε phases. The neutron diffraction results reveal that these phases can be described by domain wall lattices of different types (striped or hexagonal), and thus provide an experimental verification of recent C-IC transition theories.