  We identify a structure in the spectra of 1d lattice models of interacting
electrons, characterised by an anomalous gapped branch of elementary
excitations. Focusing on a family of Bethe ansatz solvable models, where all
excitations are stable against decay, we make a four-way classification of the
energy spectrum along with a model belonging to each class. We find in
particular that the anomalous excitation branch may switch between the spin and
charge symmetry sectors without a change in the ground state. Instead it
changes its nature by becoming inaccessible to the ground state.
