XVIII. The Pelorosaurus.--The humerus of a terrestrial reptile of enormous magnitude, has lately been discovered by Mr. Peter Fuller of Lewes, in the quarry near Cuckfield, from which many remains of the Iguanodon and Hylæosaurus were obtained in my early researches. This bone more nearly resembles the humerus of the Crocodiles, than that of the Lizards. It Is four and a half feet in length, and of corresponding proportions; it has a large medullary canal. As to the size of the animal to which it belonged, while disclaiming the idea that any certain conclusion can be drawn from a single bone, I may mention, with the view of conveying some general notion, that in a Gangetic crocodile eighteen feet long, the humerus is one foot: according to this scale the fossil animal would be eighty-one feet in length. I have proposed the name of _Pelorosaurus_ (from πἑλωρ--_pelòr_--monster), or Colossal-saurian, for this new genus of reptiles which Inhabited the country of the Iguanodon.[140] [Footnote 140: A memoir on this fossil was read before the Royal Society, Feb. 14th, 1850; an abstract has been published in the Proceedings of the Society. It is entitled, "On the Pelorosaurus; an undescribed gigantic terrestrial reptile, whose remains are associated with those of the Iguanodon and other saurians in the strata of Tilgate Forest." It will appear in the Phil. Trans. Part 11. 1850.] XIX. Silicification, _or petrifaction by flint_.--The various forms in which silex occurs have depended on its state of fluidity. In quartz crystals the solution was complete; in agate and chalcedony it was in a gelatinous state, assuming a spheroidal or orbicular disposition according to the motion given to its particles. Its condition appears also to have been modified by the influence of organic matter. In some polished slices of siliceous nodules, the transition from flint to agate, chalcedony, and crystallized quartz, is beautifully shown. The curious fact that the shells of Echinites In chalk are almost invariably filled with flint, while their crustaceous shells are changed into calc-spar, is probably in many instances to be attributed to the animal matter having undergone silicification; for the most organized parts are those which appear to have been most susceptible of this transmutation. In some specimens the oyster is changed into flint, while the shell Is converted into crystallized carbonate of lime. In a trigonia from Tisbury, formerly In the cabinet of the late Miss Benett, of Norton House, near Warminster, the body of the mollusk was completely metamorphosed into pure chalcedony, the branchiæ or gills being as clearly defined as when the animal was recent. In specimens of wood from Australia (presented to the British Museum by Sir Thomas Mitchell), which are completely permeated by silex, there are on the external surface some spots of chalcedony, that have apparently originated from the exudation of the liquid silex from the interior