The Oxford English Dictionary has these definitions as the way we currently use the words.
Lexicon
a. A word-book or dictionary; chiefly applied to a dictionary of Greek, Hebrew, Syriac, or Arabic.
The restricted use is due to the fact that until recently dictionaries of these particular languages were usually in Latin, and in modern Latin lexicon, not dictionarius, has been the word generally used.
Dictionary
a. A book which explains or translates, usually in alphabetical order, the words of a language or languages (or of a particular category of vocabulary), giving for each word its typical spelling, an explanation of its meaning or meanings, and often other information, such as pronunciation, etymology, synonyms, equivalents in other languages, and illustrative examples. Cf. lexicon n., wordbook n.