wide rounded squamosal bone at the side, which expanded towards the rear. It also shares the typical frill curvature with a top surface that is convex from side to side and concave from front to rear. Adult Achelousaurus skulls had a rugose, heavily pitted boss on the snout or nasal region, where many other ceratopsids had a horn. Such a boss is often called "pachyostotic", i.e. consisting of thickened bone. But describing it as a thick "boss" can be misleading: in fact, it forms a wide depression with a thin bone floor and irregular excavations, though it is less depressed than with Pachyrhinosaurus. The nasal boss covered about two-thirds of the top surface of the nasal bones. The boss was similar to that seen in the related genus Pachyrhinosaurus, though narrower, shorter, and less high. It covered 27% of the total skull length, was 30% longer than the nostril-eye socket distance and was about twice as long as the eye socket. Its rear edge did not reach the level of the eye socket. The nasal boss extended forward, where it fused onto the nasal and premaxilla bones (of the upper jaw) at the front of the snout, though the nasal bone itself did not fuse with the premaxilla. The boss of specimen MOR 485 furthermore had an excavation (or cavity) at its front end. The horn core that formed the boss may have developed by either becoming procurved (i.e. bent forward) during growth, like the horn of the related Einiosaurus, until it fused onto the nasal bone; or from a simple, erect horn, which later extended its base forward over the snout region, as in Pachyrhinosaurus. The nasal bone formed the top of a large bony nostril. From the rear edge of that nostril a sharp process stuck out to the front. The snout was – compared to that of Einiosaurus – relatively wide at the level of the rear nostrils. The lacrimal bone, in front of the eye socket, was thickened, mainly on the inner surface while the outer surface was featureless apart from a crater-like excavation. Adult skulls also possess large, rugose, and oval bosses on the supraorbital region above the eyes, instead of the horns of other ceratopsids. The supraorbital bosses extended from the postorbital bone forward to incorporate the triangular palpebral and prefrontal bones, and had high transverse ridges around the middle, which were thick at