Cyphellae are small round pits on the lower surface of some lichens that open to the medulla, but lined with specialized cells.  In North America, they are unique to the genus Sticta.

Cyphellae on the lower surface of Sticta fuliginosa. (A. Fischer 2/2011)

A darkfield cross-section of a cyphellum from Sticta fulginosa. Note the differentiated cells lining the cyphellum. (A. Fischer--3/2011).