From army1987's link:

Most anthropologists hold that there are no known societies that are unambiguously matriarchal, but possible exceptions include the Iroquois, in whose society mothers exercise central moral and political roles.

However, this reluctance to accept the existence of matriarchies might be based on a specific, culturally biased notion of how to define 'matriarchy': because in a patriarchy 'men rule over women', a matriarchy has frequently been conceptualized as 'women ruling over men', whereas in reality women-centered societies are - apparently without exception - egalitarian.

In other words, there isn't a trivial symmetry between those societies that are called "patriarchy" and those that are called "matriarchy".