that Euoplocephalus could be distinguished based on four traits. The premaxillae, the front snout bones, are not covered by dermal ossifications. The external bony nostrils are slit-like, face to the front and are each divided by a vertical bone sheet or septum. The beak has a width equal to or greater than the distance between the rear maxillary, upper cheek, teeth rows. The foot has three digits, metatarsals with toes. In 2013, Victoria Arbour and Phil Currie provided a differential diagnosis, setting Euoplocephalus apart from its nearest relatives. When compared with Anodontosaurus and Scolosaurus, Euoplocephalus lacks round osteoderms at the base of the squamosal and quadratojugal horns. Compared with Anodontosaurus it lacks small osteoderms at the bases of the large osteoderms on the first cervical half-ring, but, contrary to that genus, does in top view have large rounded osteoderms at the sides of the tail club. It differs from Dyoplosaurus in possessing sacral ribs that perpendicularly point outwards. It differs from Scolosaurus in possessing keeled osteoderms with a round or oval base on the top and sides of the first cervical half-ring and having a shorter rear blade of the ilium. Euoplocephalus differs from Ankylosaurus in possessing anteriorly directed external nostrils and in lacking a continuous keel between the squamosal horn and the supraorbitals. Skeleton The skull of Euoplocephalus resembles a truncated equilateral triangle when viewed from above, and is slightly wider than it is long. The largest known skull, that of specimen AMNH 5403, has a length of 411 millimetres and a width of 478 millimetres. Most skull elements are completely fused and two skull openings normally present with dinosaurs, the antorbital fenestra and the upper temporal fenestra, have closed. The skull has nineteen to twenty-four teeth in each upper jaw. The frontmost snout bone, the premaxilla, is toothless. The teeth are very small, with a maximal height and width of just 7.5 millimetres. The strongly drooping snout is blunt, wide and high, and filled with very complex air passages and sinuses, the form and function of which are not yet completely understood. Each side has two external nostrils. The lower jaw has a very concave upper rim with twenty-one teeth. At its front a short low extension is present, to provide contact with the wide predentary, the bony core of the lower horny beak, that fitted within the upper beak of the snout. As in most quadrupedal ornithischians,