Ventrally, the bony carapace shows ten thoracic vertebrae, the second through the ninth having well-developed, depressed ribs that are fused (no sutures) to the pleurals. The ribs of the first thoracic vertebra are represented by bony struts that extend posterolaterally and contact the anterior borders of the second pair of ribs. The two ribs of the ninth pair are free for most of their length and often are broken; they are slightly shorter than the eighth pair of ribs. The ribs of the tenth thoracic vertebra may be well-developed (KU 2219, 2666, 50856, spinifer, and 16528, ferox), but are usually broken off and represented only by transverse processes.
Kyphosis
Kyphosis (angular curvature of the vertebral column) or the hump-backed condition in American softshell turtles has been summarized by Nixon and Smith (1949:28). Cahn (1937:185, pl. 25e) illustrated the condition in an [473] individual of T. spinifer, and H. M. Smith (1947:119) mentioned kyphotic softshells representing the species spinifer (subspecies hartwegi and emoryi) and muticus. Neill (1951:10) mentioned two kyphotic T. s. asper and Nixon and Smith (loc. cit.) recorded the report of a kyphotic T. ferox. I have noted the condition in four muticus (subspecies muticus, KU 1959-60, 23230; INHS 2148) and seven spinifer (CNHM 22925; subspecies hartwegi, USNM 55689; subspecies spinifer, UMMZ 52948, 95615; subspecies emoryi, KU 2219, 33523, TU 16240). The smallest kyphotic specimen, a hatchling, TU 16240, has a plastral length of 3.5 centimeters. Kyphosis is to be expected in all kinds of softshells as are other abnormalities, such as albinism (reported for Lissemys by D'Abreu, 1928, and partial albinism noted in T. cartilagineus by Mohr, 1929) or congenital absence of limbs (reported by Dutta, 1931, as occurring in the genera Trionyx and Lissemys). The cause of kyphosis is not known. Smith (op. cit.:120) suggested an abnormally early fusion of the costals (= pleurals) with the ribs, and a subsequent differential rate of growth between them and the vertebral column as a hypothesis; Williams (1957:236) proposed that late retraction of the yolk mass, or retraction of an excessively large yolk mass may cause kyphosis. The cause of kyphosis may be of genetic origin or due to some environmental damage to the vertebral column prior to the cessation of growth. The variation in rate of growth of the vertebral column may produce humps of different shapes and sizes. Some of the specimens noted above (UMMZ 52948, 95615) have the carapace only slightly arched and are considered partly kyphotic. There seem to be degrees of kyphosis, a fact that should be taken into account in considering the occurrence of variation in greatest depth of shell.
Plastron
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).
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).