DALLAS, TX.- A c. 1860 Brooklyn Atlantics Baseball Card, passed down in the family of player Archibald McMahon for more than 150 years, will be on the block Thursday, July 30, 2015, as part of Heritage Auctions' Platinum Night Sports Auction, held in conjunction with the 2015 National Sports Collectors Convention in Chicago. It is expected to bring $50,000+.

"This is a seminal artifact, not just of baseball, but of American history," said Chris Ivy, Director of Sports Auctions at Heritage. "It ranks with the most significant mementos of the sport's infancy ever to surface and is, quite possibly, the only team card on Earth printed before the first drop of blood was spilled in the American Civil War."

The card is being offered by western Massachusetts resident and New York native Florence Sasso, 75, the great grand-niece-in-law of Archibald McMahon, one of the players on The Brooklyn Atlantics. The card, which was given to Sasso by her mother, the late Mildred Sasso, spent the decades with various family members before ending up with Sasso's mother, who kept it in a secret compartment in a piece of furniture the family inherited from an uncle.

Once the card was given to Sasso, she moved it from place to place — often from safekeeping in the pages of a book into another book — until she realized that the card, aside from being a link to her family history, could be quite valuable. Without children to pass it on to, Sasso has decided the time has come for a new caretaker for the artifact.

"This story of this card is also my mother's story," said Sasso. "She loved baseball and she kept it all those years. She always wanted it to take care of me and now it will."

The subject of the card is the Atlantic Base Ball Club of Brooklyn, considered the first Champion of our National Pastime, as well as the sport's first dynasty. The Atlantics served as a founding member of the sport's first organized league, the National Association of Base Ball Players, and held the Championship from 1859 through the war-shortened season of 1861 before finally surrendering supremacy to archrival Eckford of Brooklyn in 1862.

The card was passed down through the family of Archibald McMahon, an outfielder for the squad. That provenance can be traced back as far as 1928, with the obituary of 92 year old John McMahon, Archibald's brother, from which the following is excerpted:

"He also was an ardent baseball fan and had a picture in his home of the original Atlantics team, of which his brother, Archibald McMahon, was a member.

"The members of the Atlantic nine who played in a majority of games during the season of 1859 were: Boerum, catcher; M. O'Brien, pitcher; Price, Oliver and Smith on the bases; Pearce, short, and Hamilton, P. O'Brien and McMahon, in the outfield."

The Atlantics enjoyed a recent brief return to the spotlight when an 1865 team card surfaced in a small Maine auction a year ago, garnering a winning bid of $92,000.

"Certainly this is the more significant of the pair," said Ivy.

It is an interesting side note to the card that player Richard "Dickey" Pearce is widely considered one of the first professional ballplayer, a pioneer of the shortstop position, and father of what we know today as the "bunt."