In the course of my researches among British Dinosaurs the kindness of Dr. J. E. Marr and Professor T. McKenny Hughes has enabled me to investigate closely the fragmentary bone which Professor Seeley described in 1874 under the new generic name of Craterosaurus. Although the systematic position of the specimen had remained problematical from the day of its discovery, nevertheless the hope was entertained that in consequence of our present more ample knowledge of Dinosaurs and fossil reptiles in general, it might now be possible to determine the exact nature of this apparently most interesting fossil. The result was rather unexpected, for it became clear that what Seeley supposed to be the base of a cranium was nothing else than the neural arch of a dorsal vertebra showing the greatest resemblance to the corresponding element in Stegosaurus.