The thickening of the lower surface of the tectorial membrane into which the hair bundles of the outer hair cells are embedded. [ https://github.com/obophenotype/uberon/issues/1200 https://github.com/obophenotype/mammalian-phenotype-ontology/issues/2141 http://www.informatics.jax.org/accession/smith http://www.ncbi.nlm.nih.gov/pubmed/18797289 ]