Although similar in overall body size, the latter had a relatively greater skull length. Both species can be differentiated by the following characteristics: P. andrewsi – Two teeth were present at the premaxilla; the snout was low and long; the nasal horn was a single, pointed structure; the bottom edge of the dentary was slightly curved. P. hellenikorhinus – Absence of premaxillary teeth; the snout was tall and broad; the nasal horn was divided into two pointed ridges; the bottom edge of the dentary was straight. Skull The skull of Protoceratops was relatively large compared to its body and robustly built. The skull of the type species, P. andrewsi, had an average total length of nearly . On the other hand P. hellenikorhinus had a total skull length of about . The rear of the skull gave form to a pronounced neck frill (also known as "parietal frill") mostly composed of the and bones. The exact size and shape of the frill varied by individual; some had short, compact frills, while others had frills nearly half the length of the skull. The squamosal touched the (cheekbone) and was very enlarged and high having a curved end that built the borders of the frill. The parietals were the posteriormost bones of the skull and major elements of the frill. In a top view they had a triangular shape and were joined by the (bones of the skull roof). Both parietals were coossified (fused), creating a long ridge on the center of the frill. The jugal was deep and sharply developed and along with the they formed a horn-like extension that pointed to below at the lateral sides of the skull. The (tip region of the jugal) was separated from the jugal by a prominent suture; this suture was more noticeable in adults. The surfaces around the epijugal were coarse, indicating that it was covered by a horny sheath. Unlike the much derived ceratopsids, the frontal and postorbital bones of Protoceratops were flat and lacked horn cores or supraorbital horns. The (small spur-like bone) joined the prefrontal over the front of the orbit (eye socket). In P. hellenikorhinus the palpebral protruded upwards from the , just above the orbit and slightly meeting the frontal, creating a small horn-like structure. The was a near-rectangular bone located in front of the orbit, contributing to the shape of the latter. The sclerotic ring (structure that