leucabryum: which vowel joins the two halves?
heard as leucabryum
spelled leucobryum
An o. Leucobryum.
What the ear did
An unstressed vowel between two stressed chunks is the least reliable thing in any spoken word. In this name the joining vowel belongs to the Greek pattern of compounding, so it is o and never a.
L-E-U-C-O-B-R-Y-U-M
The full entry for leucobryum covers the name itself.