A warp capable humanoid species, Yridians are recognizable by their greyish skin and by a ridge which runs from their nose across the top of their heads. 
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Contact between the Vulcans and the Yridians was made at least as early as the 2150s.  However at some point subsequent to this it was thought that the Yridian species had become extinct, a belief so widespread that even the Borg, who had designated the Yridians as Species 6291, shared in it. Eventually a Starfleet exo-biologist named Rudolph Ransom discovered that species was still in existence and made contact with them, an act which led to his promotion to Captain. 
The Yridians are widely known as dealers in information, in which capacity they are willing to work with almost any other species. In 2369 a group of Yridians working for the Cardassians attacked the shuttle of Professor Galen in an attempt to steal his data on the ancient DNA code found in many Alpha Quadrant species. The Professor was killed in the action, as were the Yridians when the Enterprise-D fired upon them. Romulan monitoring of the Yridians actions led to their own involvement in the incident.  In 2369, whilst the Enterprise-D visited Deep Space Nine, a Yridian named Jaglom Shrek informed Lieutenant Worf that his father Mogh was still alive and in a Romulan prison camp. Worf forced the Shrek to accompany him into Romulan space to search for his father, only to find that although Mogh had indeed survived the Khitomer massacre he died some time previously. 
Their habit of collecting and selling information made them regulars in many varied locations such as Deep Space Nine , Dessica II , and Earth. 
The Yridians were not solely concerned with information trading, however. The Duras sisters stole a quantity of Magnesite ore from a Pakled mining colony in the Kalla system in 2370 and subsequently sold it to a group of Yridians.  The Yridians also ran convoys of supplies into the Federation-Cardassian demilitarised zone in 2370, carrying items such as hypos, plasma flares, quarantine pods. These convoys were targets for the Maquis terrorist group. 
At least some Yridians were known to be involved in shady dealings. The Enterprise-D crew encountered one who was wanted on charges within the Klingon Empire , whilst Elim Garak knew a Yridian whom he claimed wanted him dead.  In 2373 a pair of Yridians attempted to cheat on dabo in Quark's bar , though Quark didn't hold this against their species and employed a Yridian himself. 
The Rings of Paltriss were a set of 80 rings, Yridian artifacts which were considered to be extremely valuable works of art. In 2370 Quark came into possession of 42 of these rings, which he hoped to sell to the Yridian Ashrock for one hundred and ninety nine bars of gold-pressed latinum.