Old World porcupines ( Hystricidae ) have quills in clusters , whereas in New World porcupines ( Erethizontidae ) single quills are mixed with bristles , underfur , and hair . Old World porcupines ( Hystricidae ) have quills embedded in clusters , whereas in New World porcupines ( Erethizontidae ) , single quills are interspersed with bristles , underfur , and hair .