Component of the mitochondrial small ribosomal subunit (mt-SSU). Mature mammalian 55S mitochondrial ribosomes consist of a small (28S) and a large (39S) subunit. The 28S small subunit contains a 12S ribosomal RNA (12S mt-rRNA) and 30 different proteins. The 39S large subunit contains a 16S rRNA (16S mt-rRNA), a copy of mitochondrial valine transfer RNA (mt-tRNA(Val)), which plays an integral structural role, and 52 different proteins. mS40 has a zinc binding site. There are 3 mitochondrial isoforms of bS18 in mammalia, localizing to 3 distinct sites in the mitoribosome. bS18m (bs18c) binds to the same site as bacterial bS18, mS40 (bS18b, this protein) binds to a novel location of the 28S small subunit, and mL66 (bS18a) binds to the 39S large subunit. Belongs to the bacterial ribosomal protein bS18 family. Mitochondrion-specific ribosomal protein mS40 subfamily.