invalid nomen ex dissertatione. It has also been suggested that Polacanthus is simply the same species as Hylaeosaurus armatus and thus a junior synonym, but there are a number of differences in their osteology. Sometimes bones from the Hylaeosaurus material have later been made separate species. In 1928 Franz Nopcsa made specimen BMNH 2584, a left scapula referred by Mantell to H. armatus, part of the type material of Polacanthoides ponderosus. Though in 1978 synonymised with Hylaeosaurus, Polacanthoides is today considered a nomen dubium, an indeterminate member of the Thyreophora. Description Gideon Mantell originally estimated that Hylaeosaurus was about long, or about half the size of the other two original dinosaurs, Iguanodon and Megalosaurus. At the time, he modelled the animal after a modern lizard. Modern estimates range up to in length. Gregory S. Paul in 2010 estimated the length at , the weight at . Some estimates are considerably lower: in 2001 Darren Naish e.a. gave a length of . Many details about the build of Hylaeosaurus are unknown, especially if the material is strictly limited to the holotype. Maidment gave two autapomorphies, unique derived traits: the scapula did not fuse with the coracoid, even when the animal was of a considerable size; there were three long spines on its shoulder. Even these traits are not very distinctive: Mantell and Owen had attributed the lack of fusion to ontogeny and the total number of spines cannot be observed. Hylaeosaurus is often styled as a fairly typical nodosaur, with rows of armour plating on the back and tail combined with a relatively long head, equipped with a beak used to crop low-lying vegetation. In 2001 the skull and lower jaws remains were described by Kenneth Carpenter. The damaged and shifted skull elements provided little information. The quadrate is laterally bowed. The quadratojugal has a high attachment point on the shaft of the quadrate. A triangular postorbital horn was present. In 2020, it was concluded that the presumed quadrate was in fact the jugal. Several distinguishing traits were established in 2020. On the shoulder blade, there is a sharp angle of 120° between the acromion and the proximal plate. The acromial process is shelf-shaped instead of thumb-like or folded, from a point positioned at a third from the top edge projecting obliquely to below and sideways instead of strictly laterally. The top edge of the proximal plate is curved sideways. The