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Archelon
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===Plastron=== [[File:Archelon ischyros (ventral view).JPG|thumb|upright|''Archelon'' [[turtle shell#Plastron|Plastron]] at the [[North American Museum of Ancient Life]]|alt=The underside of a suspended skeleton]] 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,<ref name=wendell/> and measures (in a specimen described in 1898) {{cvt|210|cm|ft|sigfig=1}} in length.<ref name=wieland1909/> Unlike the carapace, it features striations throughout.<ref name=wieland1898>{{cite journal|last=Wieland|first=G. R.|year=1898|title=The Protostegan Plastron|journal=American Journal of Science|volume=4β5|issue=25|pages=15β20|doi=10.2475/ajs.s4-5.25.15|url=https://archive.org/details/americanjournal27unkngoog/page/n28|bibcode=1898AmJS....5...15W}}</ref> 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 two ends of the T flatten out, getting broader and thinner, as they get farther from the center.<ref name=wieland1898/> A thick, continuous ridge connects the hyoplastron, hypoplastron, and xiphiplastron. The hyoplastron features a large number of spines projecting around the circumference. The hyoplastron is slightly elliptical, and grows thinner as it gets farther from the center, before the spines erupt. The spines grow thick and narrow towards their middle portion. The seven to nine spines projecting towards the head are short and triangular. The six middle spines are long and thin. The last 19 spines are flat. There are no marks indicating contact with the entepiplastron. The hypoplastron is similar to the hyoplastron, except it has more spines, a total of 54.<ref name=wieland1898/> The xiphiplastron is boomerang-shaped, a primitive characteristic in contrast to the straight ones seen in more modern turtles.<ref name=wendell/>
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