C. Ornithopoda[86].
There is no dermal exoskeleton. The cervical vertebrae are opisthocoelous, and so are sometimes the thoracic. The limb bones are hollow and the anterior limbs are much shorter than the posterior ones. The feet are digitigrade and provided with long pointed claws. The post-pubis is long and slender and directed back parallel to the ischium;
e.g. Iguanodon from the European Cretaceous.
Order 8. Crocodilia[87].
This order includes the Crocodiles, Alligators and Garials and various extinct forms, some of which are closely allied to the early Dinosaurs.
There is always a more or less complete exoskeleton formed of bony scutes overlain by epidermal scales; these bony scutes are specially well developed on the dorsal surface but may occur also on the ventral. The vertebral column is divisible into the five regions commonly distinguishable. In all living forms the vertebrae, with the exception of the atlas and axis, the two sacrals, and first caudal, are procoelous, but in many extinct forms they are amphicoelous. The atlas (fig. 71) is remarkable, consisting of four pieces, and the first caudal is biconvex.
The teeth are, in the adult, planted in separate deep sockets. The skull is very dense and solid, and all the component bones including the quadrate are firmly united. The dorsal surface of the skull is generally characteristically sculptured. There is an interorbital septum, and the orbitosphenoidal and presphenoidal regions are imperfectly ossified. Supratemporal, infratemporal, and post-temporal fossae occur, but no interparietal foramen. In living genera there is a long secondary palate formed by the meeting in the middle line of the palatines, pterygoids and maxillae (fig. 43, A).
Cervical ribs (fig. 41, 8 and 9) are well developed, and articulate with rather prominent surfaces borne on the neural arches and centra respectively. The thoracic ribs articulate with the long transverse processes, and sternal ribs and abdominal splint ribs (fig. 46, 4) occur. The sternum is cartilaginous, and both it and the shoulder-girdle are very simple. The precoracoid is represented by merely a small process on the coracoid, while the clavicles are absent, except in the Parasuchia. In the pelvis (fig. 49) there is a large ilium, and an ischium meeting its fellow in a ventral symphysis; these two bones form almost the whole of the acetabulum. In front of the acetabulum, in the Eusuchia, projects a bone which is generally called the pubis, but is in reality rather an epipubis (fig. 49, 4), the true pubis being probably represented by a fourth element which remains cartilaginous for some time, and later on ossifies and attaches itself to the ischium. The limbs are small in proportion to the size of the body, and are adapted for swimming or for shuffling along the ground; they are plantigrade and the bones are all solid. In living forms the anterior limbs have five digits and the posterior four, the fifth being represented only by a short metatarsal. The first three digits in each case are clawed. The calcaneum has a large backwardly-projecting process.
The order Crocodilia may be subdivided into two suborders.
Suborder (1). Parasuchia.