Harmonie Universelle: De L’art de bien chanter
This comes from Harmonie Universelle, Part II, Livre VI (book 6): De L'art de bien chanter.
From p 353 on in the linked PDF, the part on embellishment (ornamentation) starts. It talks a lot about the link with poetry and rhythmical feet. From page 410 there are examples of how to embellish a simple air as done by famous musicians a little before Mersenne's time. Interestingly Mersenne first shows the great importance of respecting these rhythmical feet, but then shows how different authors ornament in different ways and have no trouble applying changes to these.
Here are the examples, so you don't have to look for them :-).
An ornamentation by Monsieur Le Bailly
A diminution by Boesset, whose original song it is. Sadly some parts missing.
Three more examples follow: