Bone tissue that forms directly within mesenchyme, and does not replace other tissues[TAO]. Intramembranous ossification is the formation of bone in which osteoblasts secrete a collagen-proteoglycan matrix that binds calcium salts and becomes calcified[GO]. Intramembranous ossification is the way flat bones and the shell of a turtle are formed[GO]. Unlike endochondral ossification, cartilage is not present during intramembranous ossification[WP]. [ http://amigo.geneontology.org/amigo/term/GO:0001957 http://en.wikipedia.org/wiki/Intramembranous_ossification ]