Iguanodon (a gigantic fossil saurian reptile, discovered by Dr. Mantell in the Wealden formation of Great Britain in 1822, and so named from the teeth resembling in shape those of the iguana. The teeth of the iguanodon resemble those of the iguana also in the elongation and contraction of the base, the expansion of the crown, the serration of the edges, and the thin coating of enamel; but the crown is relatively thicker, with a more complicated external and internal structure, and the roots are placed in separate sockets as in the crocodile. The vertebra have slightly concave articular surfaces on the body, with nearly flat sides; the neural arch of the dorsals is high and expanded, as in other dinosaurians; the anteroposterior diameter is from 4 to 4 1/2 in.; the spinal canal is completely enclosed by the neural arches; the sacral region is of considerable extent, and widely embraced by the iliac bones; in the tail the spinous processes increase for some distance below the sacrum and then diminish, and this organ was probably relatively shorter than in the iguana; the ribs are largely developed in the thoracic and abdominal regions, and connected both with the body and the transverse process of each vertebra, as in other dinosaurians and in crocodiles, and unlike the iguana and other lizards; the scapular arch is intermediate between the crocodilian and lizard type, the clavicle being more than 3 ft. long; the pelvic arch has rather a lacertian character; the thigh bones are stout, and about 3 ft. long, with the head rounded and produced, as in mammals, over the inner side of the shaft, and a singularly flattened trochanter, and must have supported the heavy body in a manner like that of the large pachyderms; the bones of the leg are robust and about 2 1/2 ft. long, and the whole extremity bears little resemblance to that of the iguana; the feet resemble those of saurians.

This reptile has been estimated by Owen as about 28 ft. in length, of which the head was 3 and the tail 13 ft.; it stood higher on the legs than any existing saurian, and was terrestrial in its habits; the worn condition of the teeth indicates that it was a herbivorous animal. It belongs to the family of dinosaurians with megalosaurus, hylceosaurns, and pelorosaurus, and is found in the Wealden and cretaceous formations. The I. Mantelli (Cuv.), from the characters of the worn dental surfaces, must have performed a true process of mastication, and the glenoid cavity must have permitted a lateral movement of the lower jaw; the large facial foramina indicate more fleshy cheeks and lips than in any existing saurians. Dr. Mantell was of opinion that it had a nasal integumental horn.

Iguanodon.

Iguanodon.