Crus commune is a part of:
(c) Semicircular canal(Ref Shambaugh, 6th ed., page 42)The posterior and superior semicircular canals fuse together to form a common crus called as "crus commune" which opens into the utricle. Because of this the three semicircular canals open into utricle by five openings instead of six.
Cochlea
Middle ear
Semicircular canal
Vestibule
Ear