Prolation: also called “mensuration canon.”
This kind of canon “could be notated by prefixing two or more different mensuration signs [like time signatures]…to a single written melody. In a mensuration canon the ratio between the two voices might be simple augmentation (second voice moving in note values twice as long as the first), simple diminution (second voice in values half as long), or some more complex ratio. Of course any of the devices just described might be used in combination. Further, the derived voice need not be at the same pitch as the original one, but might reproduce its melody at some chosen interval above or below.”
From Donald J. Grout and Claude V. Palisca, A History of Western Music, Fourth ed. (New York: Norton, 1988): 215–216.