In 2364, the Selay and the Anticans both applied for Federation membership, and as part of the process the USS Enterprise-D carried a Selay delegation to Parliament to resolve their conflict with the Anticans. (TNG: "Lonely Among Us")
According to the script, the "Selay are tall and two axhandles across the shoulders. Their reptilian heads are sinister – sleek scaled skin, lipless mouths, and beady eyes. The markings on their heads are cobra-like, and folds of skin at the neck imply a cobra hood."
The Selay species was designed by Concept Artist Andrew Probert. As such, it was one of only three alien facial designs that were conceived by Probert prior to Makeup Supervisor Michael Westmore joining the production crew of Star Trek: The Next Generation, the others being the Anticans and the Ferengi.
Although Gene Roddenberry decided (at about the start of the series) that all aliens shown on TNG were to be depicted without burying the actors under deep rubber or covering their mouths, the Selay makeup – involving a pull-over mask – broke this rule of thumb (in common with the makeup for the Anticans and Armus). (Star Trek: The Magazine Volume 2, Issue 12, p. 26) A full latex Selay mask was put up for auction in the It's A Wrap! sale and auction on eBay.