Species 312
Species 312 was the Borg designation for a race which employed a tetryon-based technology.
Based on their technology, it was speculated that the species may have been the one that visited the planet Ledos, several hundred years before 2378, and built a tetryon-based energy barrier to protect the primitive Ventu from the harmful actions of the other Ledosians. According to the Ledosian ambassador, the species that put up the barrier never returned to the planet afterwards.
Members of the species were assimilated by the Borg sometime before 2374. (VOY: "Natural Law")
This species was only mentioned in dialogue.
The Star Trek Encyclopedia, 4th ed., vol. 2, p. 179 speculates that this species could have been the Preservers.