  We study the spectroscopic evolution of superluminous supernovae (SLSNe)
later than 100 days after maximum light. We present new data for Gaia16apd and
SN2017egm, and analyse these with a larger sample comprising 41 spectra of 12
events. The spectra become nebular within 2-4 $e$-folding times after light
curve peak, with the rate of spectroscopic evolution correlated to the light
curve timescale. Emission lines are identified with well-known transitions of
oxygen, calcium, magnesium, sodium and iron. SLSNe are differentiated from
other Type Ic SNe by a prominent O I $\lambda$7774 line and higher-ionisation
states of oxygen. The iron-dominated region around 5000 \AA\ is more similar to
broad-lined SNe Ic than to normal SNe Ic. Principal Component Analysis shows
that 5 `eigenspectra' capture 75% of the variance, while a clustering analysis
shows no clear evidence for multiple SLSN sub-classes. Line velocities are
5000--8000 km/s, and show stratification of the ejecta. O I $\lambda$7774
likely arises in a dense inner region that also produces calcium emission,
while [O I] $\lambda$6300 comes from further out until 300--400 days. The
luminosities of O I $\lambda$7774 and Ca II suggest significant clumping, in
agreement with previous studies. Ratios of [Ca II]$\lambda$7300/[O
I]$\lambda$6300 favour progenitors with relatively massive helium cores, likely
$\gtrsim 6$ M$_\odot$, though more modelling is required here. SLSNe with broad
light curves show the strongest [O I] $\lambda$6300, suggesting larger ejecta
masses. We show how the inferred velocity, density and ionisation structure
point to a central power source.
