case, three quarters of the length. The second to fifth ribs project at a right angle from the midline, and, in the holotype, each measure in length. A rib increases in thickness in the vertical direction distally, as it gets farther from the midline, and the ribs are relatively larger and more well-developed than those of sea turtles. The second to fifth ribs, in the holotype, originate with a thickness of and terminate with around in thickness. The neuralia and pleuralia form highly irregular and finger-like sutures where they meet, and one plate may have lain over the other plate while the bone was still developing and malleable. The neuralia and pleuralia–the bony portions of the carapace–are particularly thin, and the ribs, especially the first rib, and the shoulder girdle are unusually heavy and may have had to carry extra stress to compensate, a condition seen in ancient ancestral turtles. Archelon has osteosclerotic structures, where the bone is dense and heavy, which probably served as ballasts in life similar to the limb bones of whales and other open-ocean animals. The carapace, in life, probably featured a row of ridges along the midline over the chest region, perhaps totaling in seven ridges, with each ridge peaking at either . In the absence of firmly jointed neck and pleural plates, the skin over the carapace was probably thick, strong, and leathery in order to compensate and properly support the shoulder girdle. This leathery carapace is also seen in the leatherback sea turtle. The spongy makeup is similar to the bones seen in open-ocean going vertebrates such as dolphins or ichthyosaurs, and was probably also an adaptation to reduce overall weight. Plastron A turtle plastron, its underside, comprises (from head-most to tail-most) the epiplastron, the entoplastron, which is small and wedged in between the former and the hyoplastron; then is the hypoplastron and finally, the xiphiplastron. The plastron, as a whole, is thick, and measures (in a specimen described in 1898) in length. Unlike the carapace, it features striations throughout. In protostegids, the epiplastron and entoplastron are fused together, forming a single unit called an "entepiplastron" or a "paraplastron." This entepiplastron is T-shaped, as opposed to the Y-shaped entoplastrons in other turtles. The top edge of the T rounds off, except at the center which features a small projection. The outward side is slightly convex and bends somewhat, away from the body. The