  More recently, N. Roy et al. [Phys. Plasmas \textbf{19}, 033705 (2012)] have
investigated the occurrence of nonlinear solitary and double-layers in an
ultrarelativistic dusty electron-positron-ion degenerate plasma using a Sagdeev
potential method. They have considered a full parametric examination on
Mach-number criteria for existence of such nonlinear excitations using the
specific degeneracy limits of Chandrasekhar equation of state (EoS) for
Fermi-Dirac plasmas. In this comment we point-out a misleading extension of
polytropic EoS to study the Fermi-Dirac relativistically degenerate plasmas.
