Who trained Samuel Clemens to become a Mississippi River pilot?

Samuel Clemens, who later became known as the writer Mark Twain, was trained to become a Mississippi River pilot by the accomplished riverboat captain Horace Bixby.1

In 1857, Clemens persuaded Bixby to take him on as an apprentice for a fee of $500. Under Bixby’s masterful instruction, Clemens studied the river and the operation of a steamboat to earn his pilot’s license. Because Bixby frequently traveled upstream for work, Clemens was also transferred to other veteran pilots during his apprenticeship, which allowed him to learn the profession more thoroughly and quickly. Clemens eventually earned his pilot's license in 1859.1