Backscattering properties of ice crystals are numerically investigated in the case of plate-like quasi-horizontally oriented crystals of cirrus clouds. In this case, a vertically oriented lidar detects the specular reflection from the clouds while a lidar with angular scanning allows one to infer the microphysical properties like the transverse shape of the crystals. It is shown that the depolarization ratio as a function of the lidar tilt reveals a step at a lidar tilt of about 30° from the vertical. This step has been observed experimentally. Appearance of this step indicates that the transverse shapes of the plate-like crystals are regular.