TAO:0002131 horizontal septum
Anatomical structure that consists of a sheet of connective tissue attached to the vertebral column and dividing the epaxial and hypaxial musculature.
Looks like a duplicate of
TAO:0000671 horizontal myoseptum
A connective tissue partition developing at the apex of the chevron-shaped myotome and separating dorsal (epaxial) and ventral (hypaxial) body wall muscle masses. Kimmel et al, 1995.
At very least the terms should be related.