IGUANODON, Mt1L—Remains of the large herbivorous reptiles of this genus have been found in Wealden and neoco mian (greensand) strata. Femora, four feet in length, showing the third inner trochanter, have been discovered. The sacrum included five, and in old animals six, vertebrae ; the claw-bones are broad, flat, and obtuse. There were only three well developed toes on the hind foot ; and singular large tridactyle impressions, discovered by Beccles in the Wealden at Hastings, have been conjectured to have been made by the Iguanodon.
With vertebrae, subconcave at both articular extremities, having, in the dorsal region, lofty and expanded neural arches, and doubly articulated ribs, and characterized in the sacral region by their unusual number and complication of structure ; with a Lacertian pectoral arch, and unusually large bones of the hind limbs, excavated by large medullary cavities, and adapted for terrestrial progression ;—the Iguanodon was distin guished by teeth, resembling in shape those of the Iguana, but in structure differing from the teeth of that and every other known reptile, and unequivocally indicating the former exis tence in the Dinosaurian order of a gigantic representative of the small group of living lizards which subsist on vegetable substances.
The important difference which the fossil teeth presented in the form of their grinding surface was pointed out by envier,* of whose description Dr. Mantell adopted a condensed view in his Illustrations of the Geology of Sussex, 4to, 1827, p. 72. The combination of this dental distinction with the vertebral and costal characters, which prove the Iguanodon not to have belonged to the same group of Saurians as that which includes the Iguana and other modern lizards, rendered it highly desirable to ascertain by the improved modes of investi gating dental structure, the actual amount of correspondence between the Iguanodon and Iguana in this respect. This has been done in the author's general description of the teeth of from which the following notice is abridged :—The teeth of the Iguanodon (fig. 77), though resembling most closely those of the Iguana, do not present an exact magnified image of them, but differ in the greater relative thickness of the crown,its more com plicated external surface, and, still more essentially, in a modification of the From and side views of a tooth of the lower jaw of internal structure, the Iguanodon, nat. size. by which the
nodon equally deviates from every other known reptile.
As in the Iguana, the base of the tooth is elongated and contracted ; the crown expanded and smoothly convex on the inner side ; when first formed it is acuminated, compressed, its sloping sides serrated, and one surface, external in the upper jaw, internal in the lower jaw, is traversed by a median longitudinal ridge, and coated by a layer of enamel ; but beyond this point the description of the tooth of the Igua nodon indicates characters peculiar to that genus. In most of the teeth that have hitherto been found, three longitudinal ridges traverse the ridged surface of the crown, one on each side of the median primitive ridge ; these are separated from each other and from the serrated margins of the crown by four wide and smooth longitudinal grooves. The relative width of these grooves varies in different teeth ; sometimes a fourth small longitudinal ridge is developed on the ridged side of the crown. The marginal serrations which, at first sight, appear to be simple notches, as in the Iguana, present under a low magnifying power (fig. 78), the form of transverse ridges, themselves notched, so as to resemble the mammilated margins of the unworn plates of the elephant's grinder ; slight grooves lead from the intempace,s of these notches upon the sides of the marginal ridges. These ridges or dentitions do not extend beyond the expanded part of the crown ; the longitudinal ridges are continued farther down, especially the median ones, which do not subside till the fang of the tooth begins to assume its subcylindrical form. The tooth at first increases both in breadth and thickness ; then it diminishes in breadth, but its thickness goes on increasing ; in the larger and fully formed teeth, the fang decreases in every diameter, and some times tapers almost to a point. The smooth unbroken surface of such fangs indicates that they did not adhere to the inner side of the maxillae, as in the Iguana, but were placed in separate alveoli, as in the Crocodile and Megalosaur ; such support would appear, indeed, to be indispensable to teeth so worn by mastication as those of the Iguanodon..