In four genera of schizotheriine Chalicotheriidae-Moropus, Tylocephalonyx, Phyllotillon, and Ancylotherium-proximal and middle phalanges of digit II of the manus fuse to form a duplex bone. Fusion of proximal and middle phalanges of digit II of the pes also occurs in many species of these genera. Radiographic and sagittal-sectional study of the fused duplex phalanges of Moropus elatus and Tylocephalonyx skinneri shows that the joint between the two bones formed fully prior to fusion but had little movement. Fusion proceeded by external enthesial interactions at the insertion of the ligament capsule and by disruption of cortical bone with trabecular bridging along the internal part of the original articular surface. Cortical disruption is more rapid on the middle than on the proximal phalangeal component of the duplex. In M. elatus, the degrees of external and internal fusion roughly correspond, and a ventral surficial groove sometimes traces the former facet, whereas in T. skinneri external fusion is much more developed than internal fusion. Phalangeal fusion seems to have strengthened the digit against strong forces applied along the medial (radial or tibial) side of the foot.