From the College Edition of Webster's New World Dictionary of the American Language © 1958 and 1953, 1954, 1955, 1956, 1957 by The World Publishing Company --
"The English Language", "I. Modern American English", "C. American English and British English", "Spelling", (2nd paragraph), p. xxvi:

5.7 Of accepted peculiarities of American spelling, the following are most notable: (1) -or for British -our in honor, favor, etc.; (2) -er for British -re in fiber, theater, etc.; (3) a single consonant for the British double consonant in traveler, traveled, wagon, riveted, etc.; (4) -s- for British -c- in offense, defense, etc.; (5) preferred -z- for British -s- in -ize, -ization, etc. (but cf. advertise, exercise, etc.); (6) e for ae or ‘ in such words as anesthesia, esthetic, etiology, medieval, etc., and in such prefixes as hemo-, hemato-, etc.; (7) bark for British barque, check for cheque, connection for connexion, cipher for cypher, draft for draught, fuse for fuze, gray for grey, hostler for ostler, jail for gaol, kilogram for kilogramme, curb for kerb, lackey for lacquey, mold for mould, molt for moult, pigmy for pygmy, plow for plough, program for programme, quartet for quartette, reflection for reflexion, story for storey, sty for stye, etc. In estimating these features, however, it must be remembered that many British authors use some American spellings, and that certain American authors occasionally prefer British spellings.