The alligator and its allies

by A. M. Reese

The five lumbar vertebræ are essentially like the thoracic except that the transverse processes, which, of course, bear no ribs, are both shorter and narrower. The postero-lateral border of the centrum of the last of these five vertebræ has a small surface for articulation with the antero-medial border of the transverse process of the first sacral vertebra.

The Sacral Vertebræ

([Fig. 18], B). These are two in number. The centrum of the first is concave in front and flat behind, instead of being convex behind, and the second is flat (instead of concave) in front, and convex behind. The neural spine and zygapophyses are as in the lumbar region. Projecting laterally from each sacral vertebra, forming a close, sutural joint with both centrum and neural arch, is a heavy bone shaped like a truncated pyramid (4); the base of the pyramid is ankylosed with the ilium. These bones seem to be much thickened transverse processes, but since they are not completely fused with their respective vertebræ and are said to ossify separately they should probably be called sacral ribs. The two sacral vertebræ do not seem to be any more closely united than are any other two vertebræ.

The Caudal Vertebræ

([Fig. 16], C). These are characterized by the entire absence of ribs, and by the presence on all but the first and the last four or five of V-shaped chevron bones. The first ten or twelve of these chevron bones articulate chiefly with the postero-ventral ends of the centra, but they also articulate with the antero-ventral ends of the vertebra behind themselves; and as they are followed caudad they seem to lie directly below the intervertebral regions and to articulate equally with the vertebræ before and behind. The chevron bones gradually diminish in size from before back. The neural processes of the first four or five caudals are broad, like those of the more anterior regions, but caudad to this point they become narrower and more pointed, though they retain the same height until about the last ten or twelve vertebræ. Towards the tip of the tail the dorsal spines diminish in height and finally disappear. The transverse processes of the first five or six of the caudals are long and narrow. They gradually diminish in length until the eighteenth caudal, back of which they are no longer to be seen. The zygapophyses are mostly about the same as in the more anterior vertebræ, but towards the posterior end of the tail the postzygapophyses come to lie between rather than above the prezygapophyses. The neural canal diminishes, of course, in size towards the tip of the tail until it is no longer present, the last five or six vertebræ consisting only of the centra.

II. The Skull.

The skull of the alligator is very massive and has several peculiarities. 1. The bones of the dorsal surface are rough and pitted, especially in old animals. 2. The jaws are enormously large in proportion to the brain cavity, and are armed with many large teeth. 3. The mandibular articulation is some distance caudad to the occipital condyle. 4. The interorbital septum is mainly cartilaginous. 5. There is a complicated system of Eustachian passages connecting with the back of the mouth by a single opening. 6. The posterior nares are placed very far back and the palate is correspondingly long.

The skull as a whole may be divided into three regions: the cranium, the lower jaw, and the hyoid; these will be described in the order given.

The Cranium.

As a matter of convenience the bones will be described as seen from the different aspects—dorsal, ventral, lateral, posterior, and in sagittal section—without particular regard to their grouping into segments or regions.