Popliteal fossa: muscles arrangement
The two Semi's go together, Semimembranosus and Semitendonosus.
The Membranosus is Medial and since the two semis go together, Semitendonosus is also medial.
Therefore, Biceps Femoris has to be lateral.
Of the semi's, to remember which one is superficial: the Tendonosus is on Top.