  Results are reported from a search for long-lived particles in proton-proton
collisions at $\sqrt{s} =$ 13 TeV delivered by the CERN LHC and collected by
the CMS experiment. The data sample, which was recorded during 2015 and 2016,
corresponds to an integrated luminosity of 38.5 fb$^{-1}$. This search uses
benchmark signal models in which long-lived particles are pair-produced and
each decays into two or more quarks, leading to a signal with multiple jets and
two displaced vertices composed of many tracks. No events with two
well-separated high-track-multiplicity vertices are observed. Upper limits are
placed on models of $R$-parity violating supersymmetry in which the long-lived
particles are neutralinos or gluinos decaying solely into multijet final states
or top squarks decaying solely into dijet final states. For neutralino, gluino,
or top squark masses between 800 and 2600 GeV and mean proper decay lengths
between 1 and 40 mm, the analysis excludes cross sections above 0.3 fb at 95%
confidence level. Gluino and top squark masses are excluded below 2200 and 1400
GeV, respectively, for mean proper decay lengths between 0.6 and 80 mm. A
method is provided for extending the results to other models with pair-produced
long-lived particles.
