The plica semilunaris is the thin upper part of the fold of mucous membrane in the supratonsillar fossa that reaches across between the two arches.
A separate fold is called the plica triangularis which runs inferoposteriorly from the posterior surface of the palatoglossal arch to cover the inferior portion of the tonsil.
This article incorporates text in the public domain from page 1138 of the 20th edition of Gray's Anatomy (1918)
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