What is the correct usage of "myriad"?

From TheFreeDictionary.com regarding myriad

Usage Note: Throughout most of its history in English myriad was used as a noun, as in a myriad of men. In the 19th century it began to be used in poetry as an adjective, as in myriad men. Both usages in English are acceptable, as in Samuel Taylor Coleridge's "Myriad myriads of lives." This poetic, adjectival use became so well entrenched generally that many people came to consider it as the only correct use. In fact, both uses in English are parallel with those of the original ancient Greek. The Greek word mrias, from which myriad derives, could be used as either a noun or an adjective, but the noun mrias was used in general prose and in mathematics while the adjective mrias was used only in poetry.

Myriad - 10,000 (from greek 'murioi')

Therefore:

10,000 men - myriad men

not:

10,000 of men - myriad of men

Of course, in modern English usage, it is often not used to mean exactly 10,000; just the way 'dozens' and 'hundreds' get used loosely, this has now come to simply mean 'a great many' in most cases.

The form remains the same, though. 'Myriad' should stand alone without 'of' following.

Hope that helps.

N.B.: Also, with regards to the question, 'myriad' can also be used to refer to something with a wide variety of elements/parts - "the myriad political scene" from OED - Here you see that 'political scene' is singular. So you could say:

The myriad things in his office - meaning 'the many items in his office.'

or something like:

The myriad apparatus/paraphernalia in his office - meaning 'the wide variety of items'