Hystricidae Erethizontidae keratin-coated spine barbed keratin-coated spine porcupine quill A keratin-coated spine that is tipped with a barb. quill UBERON:0013495 Porcupines' quills, or spines, take on various forms, depending on the species, but all are modified hairs coated with thick plates of keratin, and they are embedded in the skin musculature. Old World porcupines (Hystricidae) have quills embedded in clusters, whereas in New World porcupines (Erethizontidae), single quills are interspersed with bristles, underfur and hair.