The following is from Harvard Dictionary of Music 2nd Edit.  

From the definition of 'Discant' (bold added):

> [...] The ideal of contrary motion was mentioned in the 13th-century "Tractatus de Discantu" (in J.-A.-L. de la Fage, *Essais*, i, 358; *CS* i, 311) but became an essential characteristic of discant theory only at the end of the 13th century, when **modal rhythm was replaced by
> mensural rhythm.**

Full page of context:

[![page 263 of dictionary dirigent-discant][1]][1]

I've never heard these two terms: mensural and modal rhythm. Google gave nothing, neither did Britannica.

What do these two terms mean?


  [1]: https://i.sstatic.net/tAjLN.jpg