The Thalassans are an ethnicity or sub-race of the Andorians.They can be identified by their more wrinkled and somewhat frail appearance, but among Andorians their main characteristic are considered their bumpy antenna which they describe as sticking up from the top of their heads, but then curving so that the tips face forward. They also describe their tips as tending to look like a flower because they flare out right at the tip around the opening.
They are the most common and numerous of the Andorian races. The Thalassa system was named for them. (Last Unicorn RPG module: The Andorians: Among the Clans)