The great Mesozoic order of the _Deinosaurs_ is largely represented in the Cretaceous rocks, partly by genera which previously existed in the Jurassic period, and partly by entirely new types. The great delta-deposit of the Wealden, in the Old World, has yielded the remains of various of these huge terrestrial Reptiles, and very many others have been found in the Cretaceous deposits of North America. One of the most celebrated of the Cretaceous Deinosaurs is the _Iguanodon_, so called from the curious resemblance of its teeth to those of the existing but comparatively diminutive _Iguana_. The teeth (fig. 209) are soldered to the inner face of the jaw, instead of being sunk in distinct sockets; and they have the form of somewhat flattened prisms, longitudinally ridged on the outer surface, with an obtusely triangular crown, and having the enamel crenated on one or both sides. They present the extraordinary feature that the crowns became worn down flat by mastication, showing that the _Iguanodon_ employed its teeth in actually chewing and triturating the vegetable matter on which it fed. There can therefore be no doubt but that the _Iguanodon_, in spite of its immense bulk, was an herbivorous Reptile, and lived principally on the foliage of the Cretaceous forests amongst which it dwelt. Its size has been variously estimated at from thirty to fifty feet, the thigh-bone in large examples measuring nearly five feet in length, with a circumference of twenty-two inches in its smallest part. With the strong and massive hind-limbs are associated comparatively weak and small fore-limbs; and there seems little reason to doubt that the _Iguanodon_ must have walked temporarily or permanently upon its hind-limbs, after the manner of a Bird. This conjecture is further supported by the occurrence in the strata which contain the bones of the _Iguanodon_ of gigantic three-toed foot-prints, disposed _singly_ in a double track. These prints have undoubtedly been produced by some animal walking on two legs; and they can hardly, with any probability, be ascribed to any other than this enormous Reptile. Closely allied to the _Iguanodon_ is the _Hadrosaurus_ of the American Cretaceous, the length of which is estimated at twenty-eight feet. _Iguanodon_ does not appear to have possessed any integumentary skeleton; but the great _Hyloeosaurus_ of the Wealden seems to have been furnished with a longitudinal crest of large spines running down the back, similar to that which is found in the comparatively small Iguanas of the present day. The _Megalosaurus_ of the Oolites continued to exist in the Cretaceous period; and, as we have previously seen, it was carnivorous in its habits. The American _Loelaps_ was also carnivorous, and, like the Megalosaur, which it very closely resembles, appears to have walked upon its hind-legs, the fore-limbs being disproportionately small. [Illustration: Fig. 209.--Teeth of Iguanodon Mantellii. Wealden, Britain.]