  In quantum field theory, we learn that fermions come in three varieties:
Majorana, Weyl, and Dirac. Here we show that in solid state systems this
classification is incomplete and find several additional types of crystal
symmetry-protected free fermionic excitations . We exhaustively classify linear
and quadratic 3-, 6- and 8- band crossings stabilized by space group symmetries
in solid state systems with spin-orbit coupling and time-reversal symmetry.
Several distinct types of fermions arise, differentiated by their degeneracies
at and along high symmetry points, lines, and surfaces. Some notable
consequences of these fermions are the presence of Fermi arcs in non-Weyl
systems and the existence of Dirac lines. Ab-initio calculations identify a
number of materials that realize these exotic fermions close to the Fermi
level.
