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Page:Popular Science Monthly Volume 24.djvu/369

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355
THE IGUANODON.

pubis. . . . The post-pubis is long and slender, and directed backward alongside the ischium, as in birds, for a considerable distance beyond the ischial tuberosity. . . . M. Dollo is inclined to follow Professor Marsh in identifying the dinosaurian pubis with the pectineal process of birds, a conclusion which receives most interesting support in the valuable memoir recently published by Miss Alice Johnson, of Cambridge, on "The Development of the Pelvic Girdle in the Chick," in which it is shown that in the embryo fowl the cartilaginous representative of the pectineal process is at first much larger and more prominent in proportion to the dimensions of the pelvis than subsequently, and becomes gradually reduced as development proceeds. The peculiar form of the pelvis is, no doubt, directly connected with the muscular arrangements concerned in the erect posture, originated probably in the dinosaurians and transmitted to birds, in which it has been improved upon by the elimination, almost complete, of the original pubis through disuse."

The fore-limbs are considerably shorter than the hinder ones, and are massive and strong; and this difference in structure is cited as further though not conclusive evidence of the animal's having maintained an erect position. As further evidence in the same direction, and of the approach of the type of structure to that of birds, are mentioned the reduction of the volume of the head and thorax as compared with that of reptiles and the position of a large mass of the viscera behind the hip-joint, as in birds, whereby, with the aid of the long tail, the balancing of the head and fore-part of the body was more easily secured. The dorsal spines of the vertebrae are connected with a set of ossified ligaments binding the whole dorso-lumbar region into a rigid mass—another peculiarity in which the structure is strikingly like that of birds. The fore-limbs of the animal have five and the hind-limbs four claws, or toes, leaving a three-toed track. Here, again, is another and probably the most decisive proof that the iguanodon walked on its hind-limbs only. The feet have been compared by M. Dollo with the tridactyl Wealden foot-prints—which the iguanodon only among known Wealden dinosaurians could have made—and have been found to fit accurately. "If the animal had walked on all-fours," Mr. Moseley remarks, "it is impossible but that pentadactyl impressions should have occurred with the tridactyl, but such is not the case. Long series of the tridactyl prints are found without a trace of pentadactyl marks. The arrangement of the tridactyl tracks shows that the iguanodon walked on its hind-feet, and did not spring, like a kangaroo, with the aid of its tail. This merely dragged lightly behind, and has left no impression in connection with the foot-tracks." The first finger, or thumb, constitutes a large horny spur, the remains of which when first found were supposed to be the nose-horns of Mantell's ideal. According to M. Dollo's description, the head is relatively small, and very much compressed from side to side. The nos-