The Assimbois.—A Flat Boat belonging-to the American Fur Company, of this city, arrived here last evening fron the Company’s post on the Yellow Stone, distant 2000 miles. She brings the news of the total loss of the Company’s Steam Boat, Assiniboin, by fire. The Assiniboin was lying aground a little below the Mandn villages, where, on or about the first of June ,she took fire In the Ladies’ Cabin, and was entirely consumed. The cargo of the Assiniboin, consisting of Buffalo Robes, and valued at 10 to £80,000, was