Each pubic bone consists of an anterior or pre-pubic and a posterior or post-pubic branch, neither of which forms a symphysis. Premaxillae without teeth. With a premandibular predentary piece. Herbivorous.
The so-called pre-pubis is homologous with the pubis of most recent reptiles, and with the pectineal process of birds, while the "post-pubis" is homologous with the processus lateralis of Chelonians and Saurians, and with the "pubis" of birds. The right and left halves of the pubis remain widely asunder ventrally. In many cases the post-pubis, always directed obliquely backwards, lies closely against the shaft of the ischium, which always forms a distal syndesmosis, or a symphysis, with its fellow. The fore-limbs are usually very short, provided with five or four short and strong fingers. The hind-limbs are long and strong, mostly with three, sometimes with four functional short toes, either plantigrade (Stegosauri) or digitigrade (Ornithopoda). Femur with an inner distal, or fourth, trochanter. The dentition is of the herbivorous type, restricted to the dentaries of the mandible and to the maxillary bones, leaving the whole or the greater part of the premaxillaries free. The additional "predentary" piece of the mandible is possibly a calcified, but originally horny, pad. The teeth are greatly compressed laterally, and finely serrated, but are much ground down by use; several rows of successional teeth lie on the inner or lingual side. The skull is strongly built, with large anterior nasal openings; pre-orbital foramina very small or absent; orbits completely encircled by bones; supratemporal foramina small, lateral foramina large. Quadrate large, vertical or slanting slightly forwards. The vertebrae are solid, not hollow; sacrum consisting of four, five, or more vertebrae; ribs bifurcated, the capitula carried either by the centra, or moved up to the diapophyses of the neural arches; chevron-bones numerous, and frequently long, especially on the anterior half of the long and heavy tail.
Orthopoda occur from the Lias to the Upper Cretaceous, both in Europe and in North America. The name Orthopoda, invented by Cope in 1866, is appropriate for obvious reasons; it comprises the Stegosauri and Ornithopoda of Marsh (1881). The latter term is not very fortunately chosen, considering that the whole hind-limb of the Theropodous Compsognathus is far more ornithic than that of any three-toed Ornithopoda, in which the tarsalia rarely fuse with the tibia and never with the metatarsals. To apply the term Ornithopoda to the whole order is quite unjustifiable, unless it is meant to apply to the strikingly bird-like configuration of the pelvis.
Sub-Order 1. Stegosauri.–The fore- and hind-feet are plantigrade, or nearly so, the metapodials being but little elongated, with more than three functional digits. The bones of the limbs are solid. The ribs of the trunk are bifurcated, and are carried by the diapophyses of the neural arches. The body, especially the back, is protected by dermal bony plates, which are not connected with the internal skeleton.
Scelidosaurus harrisoni. One nearly complete skeleton, about 11 feet in length, from the Lias of Lyme Regis. About twenty-four pre-sacral vertebrae, of which six or seven belong to the neck, four sacral and about forty caudal vertebrae. Four fingers, four toes, with 2, 3, 4, 5 phalanges, the fifth metapodials being quite vestigial; the hallux and pollex are very short, so that the foot at least is functionally tridactyle. The tarsal bones remain separate. The head is very small. Two rows of ridged bony plates extend from the neck over the back, and converge into one row upon the long tail; smaller plates, arranged in many rows, seem to have protected the sides and under parts. Hylaeosaurus and Polacanthus of the English Wealden are allied forms.
Stegosaurus, with several species from the Upper Jurassic of Colorado and Wyoming, and others, e.g. S. armatus (= Omosaurus), from the Kimmeridge Clay of Wiltshire in England. The head is relatively very small, and the brain is surpassed several times in thickness by the huge sacral swelling of the spinal cord. Teeth numerous and small. All the cervical and trunk-vertebrae carry bifurcated ribs, those of the trunk being carried entirely by the very high neural arches. The fore-limbs are only about half the length of that of the hind-limbs, so that these creatures, which were undoubtedly quadrupedal, must have had a very peculiar gait, standing with the head, neck, and shoulders much lower than the arched back and pelvic region. The ulna has a strong olecranon; the hand has four functional fingers. The pre-acetabular portion of the ilium is much elongated; the pre-pubic branch stands horizontally, while the post-pubis is closely adpressed to the ischium. The astragalus is fused with the tibia, the calcaneum with the fibula. The foot has only three short toes, protected, like the fingers, by hoofs. The dorsal dermal armature consists of very high, crest-like plates. S. ungulatus of North America has a computed length of 28 feet, with the hind-limbs about 7 feet long. This creature was nearly 10 feet high, when measured from the ground to the tips of the dermal crests on the middle of the back. These bony, laterally compressed plates are themselves nearly 3 feet high, and are replaced, on the hinder portion of the tail, by several pairs of pointed spikes about 2 feet in length.
Fig. 100.–Skeleton and dermal armour of Stegosaurus ungulatus. × 1⁄60. (After Marsh.)
Sub-Order 2. Ornithopoda.–The hind-limbs are distinctly digitigrade, usually with only three functional toes, protected by claws. The long bones are hollow. Femur with a long fourth trochanter. Without dermal armour-plates.
Camptosaurus.–Several species, up to 10 feet in length, from the Upper Jurassic and the Wealden of North America and England. Five fingers, with 2, 3, 3, 3, 2 phalanges and four toes, with 2, 3, 4, 5 phalanges, but the hallux is much shortened and does not touch the hard ground; astragalus and calcaneum separate.