Petersburg, Virginia. Unidentified  Man?

Although "unidentified" on official sites, this officer is Maj. Simon Forrester Barstow (1817-1882) of Boston. He was Meade's Assistant-Adjutant-General from after Gettysburg until the end of the war. Not my identification alone; others have made the connection in various settings, including Find A Grave. This page collects and expands the Barstow references. 
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LC 00717 & 00718 composite. "Petersburg, Virginia. Unidentified man," photographer unknown. Simon Barstow was photographed at headquarters, Army of the Potomac, before Petersburg.
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MOLLUS Collection, Vol. 44, 2165




​A carte-de-visite (right) sold by Cowan's Auctions in 2015 correctly identifies the individual as Barstow. The CDV looks like an off-print of the above photograph. 
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CDV identified as Simon Forrester Barstow (private collection)
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LC 00717 detail. Meade's officers took a collection in fall 1863 to create a medallion for the army staff to wear. It served a practical purpose of identifying the wearer as official during action.
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Annotated version of the Gardner photograph from the collection of the Smithsonian American Portrait Gallery. Barstow seated at far right is labeled below.