. Carnegie Institution of Washington publication. CHELONIIDAE. 213 obtuse, and along the hinder third is somewhat dentated. Anteriorly, the upper surface is furnisht with ridges which are sharp and prominent as they approach the free inner border, but which become obsolete in the opposite direction. These ridges seem to diverge from the anterior buttress of the hyoplastron (fig. 270, C), if such existed. At the anterior end of the bone the inner border turns slightly outward and, at an angle of about 750, meets the sutural border for articulation with the epiplastron (fig. 270, A to B). These
. Carnegie Institution of Washington publication. CHELONIIDAE. 213 obtuse, and along the hinder third is somewhat dentated. Anteriorly, the upper surface is furnisht with ridges which are sharp and prominent as they approach the free inner border, but which become obsolete in the opposite direction. These ridges seem to diverge from the anterior buttress of the hyoplastron (fig. 270, C), if such existed. At the anterior end of the bone the inner border turns slightly outward and, at an angle of about 750, meets the sutural border for articulation with the epiplastron (fig. 270, A to B). These bones were not joined by squamosal suture, as they are in the Cheloniidae and the Chelydridae, but by dentated suture. The lower surface of the bone is smooth and convex, especially transversely. The upper surface is concave. At the anterior end of the bone, on the upper surface, 190 mm. behind the anterior end (fig. 270, C), is a slight elevation, as if it were the base of an axillary buttress. The lower surface of the bone just behind this point also indicates that the plastron rose toward the carapace. The portion of hypoplastron has a length of 155 mm. and a width of 125 mm. Anteriorly the suture for the hyo- plastron is missing. The inner border is subacute and smooth, thus differing from the corresponding border of the hyoplastron. The suture for the xiphiplastron runs at right angles with the midline. It shows that the two bones were firmly united by coarse interdigitating teeth. Toward the outer border the bone has a thickness of about 13 mm., but this diminishes slowly toward the inner border. As the outer border is approacht, the bone begins to ascend in a decided manner, as if approaching the carapace (fig. 270, D). If this conclusion is correct, as indicated by both Inn- plastron and hypoplastron, these bones were, relatively to their length, very narrow; and there is indicated a wide fonta- nel, that extended from the entoplastron to the xiphiplastra. It seems impr
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