A mammal with spines or quills in addition to regular hair. The 23 species are classified in the order Rodentia and consist of two families: the Hystricidae, or Old World porcupines (11 species), and the Erethizontidae, or New World porcupines (12 species(see table)). Although traditionally considered to be related, the two families of porcupines may have evolved independently from different, unrelated ancestors. The infraorbital foramen is usually larger than the foramen magnum, and the zygomatic arch is also very large. Members of both families have a dental formula of I 1/1, C 0/0, Pm 1/1, M 3/3 × 2 = 20. Porcupines are herbivorous, terrestrial, and generally nocturnal.