Early in 2006, I was working as a volunteer proofreader for the Project Gutenberg digital library: I noticed that the Project Gutenberg library included some public domain classical editions comparable in scope to my own. Prompted by this, I decided that I would publish my edition online in order to make it instantly accessible free of charge to anyone wishing to use it. This seemed in every way preferable to seeking out a university press, going through the time-consuming process of seeking the necessary grants to subsidize publication, in order to produce a printed book so expensive that no student and not many libraries could afford to purchase a copy. Nature of this edition In essence, this is a corrected version of the original typescript. Typing errors have been corrected, and minor errors have been set right. All statements made and conjectures proposed should be considered to have been made in 1985. The HTML and Text versions of this edition This digital edition is being made available in two versions. The _HTML version_ takes advantage of the Unicode character set to present Greek passages using the Greek alphabet, and to present certain other special characters, such as the macron. It also offers hyperlinks from the table of contents and from the indices to the relevant sections of the edition. Popular and useful as HTML is, it does not offer the universality of ASCII text. Essentially every computer can display plain ASCII text correctly. The _Text version_ is presented so that the edition can be read on any computer, large or small, new or old. However, this portability comes at a price. The ISO 8859-1 ASCII character set does not include the Greek alphabet, nor does it include certain special characters which form part of this edition. Therefore, the Text version of this edition presents Greek passages transliterated into the Latin alphabet. Similarly, in the textual apparatus any capital letter occurring in the report of a manuscript should be considered to be that letter in lower case, with a macron (dash) above. When the textual apparatus reports a manuscript correction where the original reading is no longer legible, the HTML version underlines the corrected letters, but the Text version uses capitalization. For example, the Text version reports "facTisque _B2c_" at iii 25: a later hand in _B_ has erased the original fourth letter, and has replaced it with "t". In the commentary, when metre is being discussed and a Latin word is quoted, any vowel in that word which is capitalized is long, and any vowel which is not capitalized is short. I have occasionally pointed out explicitly that a word is metrically inconvenient because it has a series of short vowels: in the HTML edition, because the actual letters are marked short, these statements will appear to be redundant.