F.M., or f.m., foramen magnum.
"Text Book of Biology, Part 1: Vertebrata" by H. G. Wells
Sudden death may occur from crowding of the cerebellum into the foramen magnum.
"Manual of Surgery Volume Second: Extremities--Head--Neck. Sixth Edition." by Alexander Miles
The exoccipitals extend around the sides of the foramen magnum and meet dorsally.
"A Revision of Snakes of the Genus Conophis (Family Colubridae, from Middle America)" by John Wellman
The foramen magnum is more triangular.
"The Cambridge Natural History, Vol X., Mammalia" by Frank Evers Beddard