Srednicki assumes in chapter 33 $d=3$ space dimensions. Otherwise (33.11) doesn't make sense. The Lorentz group is in this case a direct product of two $SO(3)$ groups, hence the irreducible representations are labelled by two generalized spin indices, one for each copy of the $SO(3)$. (You can get the latter in terms of a sub $SO(2)$ and corresponding ladder operations.)
However, it is misleading to think of the irreducible representations of the Lorentz group as ''particle representations''. Particles are associated with unitary positive energy representations of the Poincare group, not with arbitrary representations of the Lorentz group.