The entries for both plants have a decent collection of pics from various peeps/plants that one can see are 2 different plants after seeing the amalgam of just foliage pics of both, but having trouble picking or describing a feature that lets my eyes know that.
Closeups of the Crassula show pores (?) that look like dots on the foliage. The Cotyledon pics didn't show that. Is that a diagnostic feature, or happenstance among the particular pics collected for both here so far?
