You heard it right! “BaseS” in the plural. I knew about globalchant.org, created by Jan Koláček, a doctoral student at Prague’s Charles University who was on my brother’s panel at Kalamazoo this year. Great chap, and, though it’s not my area of expertise (to put it mildly), the site certainly looks impressive. And I also came across Cantus, run by Debra Lacoste at Western Ontario; it also looks very good. So, for those of you who are musicologist types (you know who you are), I hope these prove useful.
[Above is the manuscript image from Global Chant Database. Don’t ask me to identify it, as I am woefully ignorant in that regard (except regarding some crusade psalms and sequences, from a non-musical standpoint):]