  The statistics of strongly lensed arcs in samples of galaxy clusters provide
information on cluster structure that is complementary to that from individual
clusters. However, samples of clusters that have been analyzed to date have
been either small, heterogeneous, or observed with limited angular resolution.
We measure the lensed-arc statistics of 97 clusters imaged at high angular
resolution with the Hubble Space Telescope, identifying lensed arcs using two
automated arc detection algorithms. The sample includes similar numbers of
X-ray selected (MACS) and optically selected (RCS) clusters, and spans cluster
redshifts in the range 0.2 < z < 1. We compile a catalogue of 42 arcs in the
X-ray selected subsample and 7 arcs in the optical subsample. All but five of
these arcs are reported here for the first time. At 0.3 < z < 0.7, the X-ray
selected clusters have a significantly higher mean frequency of arcs, 1.2+/-0.2
per cluster, versus 0.2+/-0.1 in the optical sample. The strikingly different
lensing efficiencies indicate that X-ray clusters trace much larger mass
concentrations, despite the similar optical luminosities of the X-ray and
optical clusters. The mass difference is supported also by the lower space
density of the X-ray clusters, and by the small Einstein radii of the few arcs
in the optical sample. Higher-order effects, such as differences in
concentration or substructure, may also contribute.
