The lattice parameters of a title crystal, TMACA, were measured from a room temperature to 405 K by the X-ray single-crystal Bond method. The high-temperature paraelectric phase should have the monoclinic symmetry (space group P2/c). Some peculiarities on dielectric behaviour of the crystal were explained. The hypotheses on incommensurability of the paraelectric phase as well as on the parent normal (trigonal) phase were presented.