The ratio of the muon flux density to charged particle flux density at distances of 300 and 600 m from the shower axis ($rhom(300)/rhos(300)$ and $rhom(600)/rhos(600)$) is measured. In addition, the energy dependence of $rhom(1000)$ is analysed for showers with energies above $10^{18}$ eV. A comparison between the experimental data and calculations performed with the QGSJET model is given for the cases of primary proton, iron nucleus and gamma- ray. We conclude that the showers with $Ege3times10^{18}$ eV can be formed by light nuclei with a pronounced fraction of protons and helium nuclei. It is not excluded however that a small part of showers with energies above $10^{19}$ eV could be initiated by primary gamma-rays.
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