The jury is not necessarily in on the proper use of 'myriad'. For years we heard 'a myriad, and 'myriads of', then 'myriad' started standing alone. 'A myriad stars' or 'myriads of stars' is music to our ears. Most people still use these two phrases. Some muck it up completely and say 'a myriad of stars'.
Once my mentor declared the acceptable form among widely published authors, like him and his novelist friends, is 'myriad', my love of myriad was ruined forever.
Over time, I became a believer of the single myriad. I had to do it on my own terms, however, with a comparison I could accept. Here's how it goes. Myriad is a word indicating vast numbers, like the word 'thousand'. Would we say 'a thousand of stars? No, we'd drop the 'of'. So, I'm okay with "a myriad stars'. Likewise, as in the plural, we would say 'thousands of stars' and drop the 'a'. So, I'm okay with 'myriads of stars'.
If you find yourself giving pause the next time you use 'myriad', you can blame me. Then, you'll most likely sort it out for yourself. Some of you may choose to research it, too. I didn't recently. I'm pretty sure that jury is, indeed, still out. One of the things I love most about language is that it changes, and, eventually, the rules of usage actually catch up with the usage.