  The aim of this letter is to check the ultra-relativistic limit of a recent
model proposed by Pennisi and Ruggeri [Ann. Phys. 377, 414 (2017)] for a
relativistic gas with internal structure. Considering an Eulerian fluid we
prove that there exists a critical value of the degree of freedom such that for
smaller values of this quantity the ultra relativistic limit of the energy of a
gas with structure is the same as the Synge energy for monatomic gases, while
for larger degrees of freedom the energy increases with the degree of freedom
itself. The limiting equations are accompanied with the explicit expression of
the characteristic velocities of the hyperbolic system.
