  We present a study of 28 Type I superluminous supernovae (SLSNe) in the
context of the ejecta mass and photospheric velocity. We combine photometry and
spectroscopy to infer ejecta masses via the formalism of radiation diffusion
equations. We show an improved method to determine the photospheric velocity by
combining spectrum modeling and cross correlation techniques. We find that Type
I SLSNe can be divided into two groups by their pre-maximum spectra. Members of
the first group have the W-shaped absorption trough in their pre-maximum
spectrum, usually identified as due to O II. This feature is absent in the
spectra of supernovae in the second group, whose spectra are similar to
SN~2015bn. We confirm that the pre- or near-maximum photospheric velocities
correlate with the velocity gradients: faster evolving SLSNe have larger
photosheric velocities around maximum. We classify the studied SLSNe into the
Fast or the Slow evolving group by their estimated photosheric velocities, and
find that all those objects that resemble to SN~2015bn belong to the Slow
evolving class, while SLSNe showing the W-like absorption are represented in
both Fast and Slow evolving groups. We estimate the ejecta masses of all
objects in our sample, and obtain values in the range of 2.9 ($\pm$0.8) - 208
($\pm$61) $M_\odot$, with a mean of $43 (\pm 12)~ M_\odot$. We conclude that
Slow evolving SLSNe tend to have higher ejecta masses compared to the Fast
ones. Our ejecta mass calculations suggests that SLSNe are caused by energetic
explosions of very massive stars, irrespectively of the powering mechanism of
the light curve.
