After signing Eric Wright and Maurice Leggett and re-signing Brandon McDonald, the Detroit Lions have quite a bit of depth at cornerback for next season. They could certainly still use an upgrade in talent, but right now they have a good combination of players that should allow them to get by with what they have.
It's still possible the Lions will add another cornerback in the near future, but it won't be Richard Marshall. From Adam Schefter:
Cardinals reached agreement with former Panthers CB Richard Marshall on a one-year contract.less than a minute ago via UberSocial for BlackBerry 
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Adam Schefter
AdamSchefter
Marshall was a popular name amongst fans as a possible addition for the Lions. With him off the table, I'm starting to think the most likely cornerback to be signed, if any, is Chris Houston. Bringing him back along with adding Wright would not be a bad set of moves in my mind. Neither player is an elite corner, but the cornerback position would be much improved compared to last season.