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Five mesomorphic phases are described in several disc-like liquid-crystals. They are hexaalkanoyloxy benzenes, triphenylenes or truxenes, hexa-alkoxy triphenylenes, hexa-benzoates of triphenylene. Several of these compounds exhibit a complex polymorphism. Informations on the structures of these phases have been obtained by means of X-Ray diffraction; except a ND namatic phase all the others one are columnar phases. At the same time a systematic study has been performed by the well known method of isomorphy. All these results allow us to propose a general classification of these phases.

The X-ray diffraction patterns of powder samples of the fluid mesophase of the hexaalkoxybenzoates of triphenylene which is a disc-like molecule are similar to the diffraction patterns of non orientated nematic phase of rod-like molecules. Furthermore, we give evidence for a uniaxial character of this anisotropic medium with a negative diamagnetic anisotropy, the flat molecules are orientated parallel each other by a rotating magnetic field. In addition, the X-ray patterns of orientated samples of the hexyloxy compound revealed pretransitional effects in this nematic phase of disc-like molecules (ND). This short-range translational order, sort of skewed cybotactic groups originates from the existence of a low temperature columnar phase with an original tilted packing here described

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