  First evidence of a structure in the $J/\psi \varLambda$ invariant mass
distribution is obtained from an amplitude analysis of $\varXi_b^- \to J/\psi
\varLambda K^-$ decays. The observed structure is consistent with being due to
a charmonium pentaquark with strangeness. Its mass and width are determined to
be $4458.8\pm2.9\,^{+4.7}_{-1.1}$MeV and $17.3\pm6.5\,^{+8.0}_{-5.7}$MeV, where
the quoted uncertainties are statistical and systematic, respectively. The
structure is also consistent with being due to two resonances. In addition, the
narrow excited $\varXi^-$ states, $\varXi(1690)^-$ and $\varXi(1820)^-$, are
seen for the first time in a $\varXi_b^-$ decay, and their masses and widths
are measured with improved precision. The analysis is performed using $pp$
collision data corresponding to a total integrated luminosity of 9fb$^{-1}$,
collected with the LHCb experiment at centre-of-mass energies of $7$, $8$ and
$13$TeV.
