The dorsal surface of the bony carapace of American trionychids consists of a nuchal, seven or eight pairs of pleurals, and seven or eight, rarely nine, neurals ([Fig. 16]). The lateral parts of the nuchal overlie the second pair of ribs. The distal parts of the second through the ninth pair of ribs extend laterally beyond the lateral edges of the pleurals. There are no marginal ossifications. The posterior part of the bony carapace bears blunt, rounded or ovoid to linear, prominences mostly on the last pair of pleurals principally on large females of spinifer and ferox; I have seen only one adult male (stuffed, MCZ 46633) having a semblance of welts on the bony carapace. The nuchal, pleurals and neurals are sculptured.
As growth proceeds, the single, transversely-oriented, fontanelle of young turtles that separates the nuchal from the first neural and first pair of pleurals divides into two fontanelles that generally decrease in size and finally disappear. Occasionally only one (unilateral) large fontanelle is present (USNM 54734, muticus). The largest specimens noted that retain fontanelles are a ferox (USNM 029474) having a plastron 24 centimeters long, and a spinifer (USNM 54731) having a plastron 20 centimeters long. The fontanelles probably are present in some larger individuals.
Fig. 16. Carapace of Trionyx spinifer (a), and sketches of posterior parts of carapaces (b-i) of three American species, showing number and variation in arrangement of neurals and pleurals (not to scale; seventh neural, n7, and pleural, p7).
a. KU 2226, Lewisville, Lafayette County, Arkansas (× 1/3); sculpturing incompletely shown. Labels: r, ribs; nu, nuchal; n, neurals 1-7; p, pleurals 1-7.
b. ferox, USNM 60496, Auburndale, Polk County, Florida.
c. muticus, KU 1964, Doniphan Lake, Doniphan County, Kansas.
d. spinifer, USNM 100380, Plaquemine, Iberville Parish, Louisiana.
e. muticus, TCWC 7260, Red River, 8 mi. NW Ringgold, Montague County, in Clay County, Texas.
f. spinifer, USNM 59266, Homer, Winona, Minnesota.