The ventral surface of the quadrate that articulates with the mandible is composed of a lateral condyle and a medial articular surface. The condyle and medial articular surface are separated by a furrow. On skulls of _ferox_ and _spinifer_ the lateral condyle, which is not conspicuously tapered posteriorly, is slightly larger than the medial articular surface, and the furrow is shallow. On skulls of _muticus_, the lateral condyle is conspicuously tapered posteriorly, is slightly smaller than the medial articular surface, and the furrow is deep. _Contact of Maxillaries Above Premaxillaries_ The contact of the maxillaries above the premaxillaries is of diagnostic value in distinguishing skulls of _ferox_ and _spinifer_ from those of _muticus_. I have seen no skulls of _muticus_ on which the maxillaries were in contact, and no skulls of _ferox_ on which the maxillaries were separated. Stejneger (1944:19), however, reported a skull of _muticus_ (USNM 102677) having the maxillaries in contact. Maxillaries are in contact (sometimes just barely) in 65 of 74 skulls of _spinifer_ (88%); the premaxillaries are separated on nine skulls (12%). _Carapace_ 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. [Illustration: 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.