The plastron is united to the carapace by ligamentous tissue and is somewhat flexible anteriorly and posteriorly. Anteriorly the plastron is somewhat hingelike and may contact the anteriormost edge of the carapace. The bony elements are reduced. There is usually a median vacuity, which is relatively smaller in larger specimens and may be divided into two vacuities (a posteromedial and an anteromedial) by the medial juxtaposition of the hyo-hypoplastra, especially in _muticus_. Williams and McDowell (1952) have recommended a change in nomenclature for some of the plastral bones on the basis of reinterpretation of their homologies. The nine plastral bones include: an anterior pair of preplastra (= epiplastra, _auct._); an unpaired, median bone, representing fused epiplastra (= entoplastron, _auct._), hereafter referred to as the epiplastron; a pair of hyoplastra; a pair of hypoplastra; and, posteriorly, a pair of xiphiplastra (Fig. 17). Siebenrock's (1902) synopsis of living trionychids was based entirely on plastral characters. He distinguished between _muticus_ and _spinifer_ principally by the shape of the epiplastron; _T. ferox_ was not considered different from _spinifer_. The median angle formed by the boomerang-shaped epiplastron is obtuse and somewhat greater than 90 degrees in _muticus_ (Fig. 17a); the angle of the epiplastron in _spinifer_ and _ferox_ is smaller than in _muticus_ and forms an approximate right angle (Fig. 17b). Williams and McDowell (_op. cit._:277, Pl. 1, Fig. 3) presented an illustration of the anterior plastral elements of an adult _T. ferox_. Siebenrock provided illustrations of the plastrons of _muticus_ (_op. cit._:823, Fig. 5) and _spinifer_ (_op. cit._:830, Fig. 10). [Illustration: FIG. 17. Plastron of _Trionyx muticus_ (a) and _T. spinifer_ (b); sculpturing of callosities incompletely shown. ep, epiplastron; hp, hyoplastron; hyp, hypoplastron; pp, preplastron; xp, xiphiplastron. a--KU 1868, White River, Devall's Bluff, Prairie County, Arkansas (× 2/3); b--KU 1869, same locality (× 2/3).]