During a recent stay in London the kindness of Dr. A. S. Woodward enabled me to study some of the splendid Dinosaurian remains in the British Museum.

Having principally occupied myself till now with Ornithopodous Dinosaurs, first of all Hypsilophodon attracted my attention, and my expectation that this type would prove to be the clue for the understanding of all the other Orthopoda has been perfectly fulfilled.

Hypsilophodon was described and figured at various times by Owen, Huxley, and Hulke; a restoration of this animal was given by Marsh in the Geological Magazine for 1896 (p. 6, Fig. 2), and the complete bibliographical list concerning this Dinosaur is compiled in my paper “Synopsis und Abstammung der Dinosaurier” (Földtani Közlöny, 1901, Budapest).

In consequence of our more recent knowledge of Dinosaurs in general I managed to detect some new points of remarkable interest.

Mandible. What Hulke, in describing the Hypsilophodon skull, No. 110, supposed to be the parietal, frontal, and post-frontal bones (Phil. Trans., 1882, pl. lxxi, fig. 1, pa., fr., ps.f.), turned out to be the outer view of a complete right mandibular ramus, so that the parietal changes into an articular, the frontal becomes the dentary, and the post-frontal the coronoid bone. This piece (p. 207, Fig. 4) is, in fact, the finest mandibulum of Hypsilophodon I have seen in the whole collection, and as such worthy to be refigured.

The general outline of the mandibulum, with its strongly abbreviated post-coronoidal part and its blunt processus coronoideum, reminds one somewhat of the under jaw of Placodus gigas, though it differs of course in nearly every detail, being built up after the Iguanodon type.