These are five in number, and are precisely like the posterior thoracic vertebrae, except in the fact that the transverse processes have no facets for the articulation of ribs. THE SACRAL VERTEBRAE. These are two in number, and while the centrum of the first is concave in front (fig. 42, B, 6) and nearly flat behind, that of the second is flat in front and concave behind. Each has a pair of strong =ribs= (fig. 42, B, 4) firmly ankylosed in the adult with a wide surface furnished partly by the centrum, partly by the neural arch. The distal ends of these ribs are united with the ilia. The character of the neural spines and zygapophyses is the same as in the thoracic vertebrae. THE CAUDAL VERTEBRAE. These are very numerous, about thirty-four in number. The first differs from all the other vertebrae of the body in having a biconvex centrum. The succeeding ones are procoelous and are very much like the posterior thoracic and lumbar vertebrae, having high neural spines and prominent straight transverse processes. They differ however in having the neural spines less strongly truncated above, and the transverse processes arise from the centra and not from the neural arches. When followed further back the centra and neural spines gradually lengthen while the transverse processes become reduced, and after the twelfth vertebra disappear. Further back still the neural spines and zygapophyses gradually become reduced and disappear, as finally the neural arch does also, so that the last few vertebrae consist simply of cylindrical centra. Each caudal vertebra, except the first and the last eleven or so, has a =V=-shaped =chevron bone= attached to the postero-ventral edge of its centrum. The anterior ones are the largest and they gradually decrease in size till they disappear. B. THE SKULL[90]. The skull of the Crocodile is a massive depressed structure presenting a number of striking characteristics, some of the more important of which are:-- 1. All the bones except the mandible, hyoid, and columella are firmly united by interlocking sutures. In spite of this, however, growth of the whole skull and of the component bones goes on continuously throughout life, this growth being especially marked in the case of the facial as opposed to the cranial part of the skull. 2. All the bones appearing on the dorsal surface are remarkable for their curious roughened and pitted character; this feature is prominent also in many Labyrinthodonts. 3. The size of the jaws and teeth is very great. 4. The mandibular condyle is carried back to some distance behind the occipital condyle. 5. The occipital plane (see p. 386) of the skull is vertical. 6. The length of the secondary palate is remarkably great, and the vomer takes no part in its formation. 7. The posterior nares are placed very far back, the nasal passages being as in mammals separated from the mouth by the long secondary palate.