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.]

Another remarkable group of Reptiles, exclusively confined to the Cretaceous series, is that of the Mosasauroids, so called from the type-genus Mosasaurus.  The first species of Mosasaurus known to science was the M.  Camperi (fig. 210), the skull of which—­six feet in length—­was discovered in 1780 in the Maestricht Chalk at Maestricht.  As this town stands on the river Meuse, the name of Mosasaurus ("Lizard of the Meuse”) was applied to this immense Reptile.  Of late years the remains of a large number of Reptiles more or less closely related to Mosasaurus, or absolutely