Fig. 4. The femoro-tibial joint of the left hind leg of an adult female Prosarthria teretrirostris. (A) View from the anterior surface. The anterior semi-lunar process is a thin curved structure forming a dark groove in the femur. The coverplate is sculpted so that the pivot of a tibial horn with the incurving part of the anterior semi-lunar process is visible. The extensor tibiae muscle inserts on the dorsal rim of the tibia and the flexor tibiae muscle around a ventral U-shaped rim of tibial cuticle. (B) Ventral view with the tibia extended. The ventral femur has infoldings that form an internal lump over which the tendon of the flexor tibiae muscle slides (see Fig. 5B). The membrane surrounding the tendon is folded to form a pocket. Hair rows on the proximal tibia can be seen. (C) Dorsal view of the tibia and its articulation with the femur with the joint flexed. The pivot between the femur and the tibia is seen as a pair of close appositions between the two semi-lunar processes of the femur and the horns of the tibia.