In the first book of the series, Arthur's Nose (1976), he has a long, aardvark-like nose, but in later books, his face becomes more rounded.
ɪnðə fˈɜːst bˈʊk ʌvðə sˈɪɹiz, ˈɑːɹθɚz nˈoʊz nˈaɪntiːnhˈʌndɹɪd sˈɛvənti sˈɪks, hiː hɐz ɐ lˈɔŋ, ˈɑːɹdvɑːɹklˈaɪk nˈoʊz, bˌʌt ɪn lˈeɪɾɚ bˈʊks, hɪz fˈeɪs bɪkˌʌmz mˈoːɹ ɹˈaʊndᵻd.