Largest freshwater turtle in North America (record is 80 cm CL). Wild individuals rarely reach 70 kg (155 lb), but captives may exceed 100 kg (220 lb). Carapace sculptured, dull black or brown, and strongly serrate posteriorly. Three carapacial keels formed by pyramid-shaped elevations of vertebral and pleural scutes. Plastron reduced and cross-shaped. Head cannot be retracted within shell. Chin, throat, and neck decked with long, pointed tubercles. Tail nearly as long as shell. Male larger than female. Male cloacal opening extends about twice as far beyond back edge of shell (mean = 19.9 cm).