The calcar, also known as the calcaneum, is the name given to a spur of cartilage arising from inner side of ankle and running along part of outer interfemoral membrane in bats, this is to help spread the interfemoral membrane, which is part of the wing membrane between the tail and the hind legs.
This membrane can form a basket or pouch to help catch and hold insects captured in flight. The oldest known ancestor to present day bats, Icaronycteris index, apparently did not have a calcar or spur as evidenced by fossil remains.
Calcar (Femorale) also refers to the dense, vertically orientated bone present in the posteroemedial region of the femoral shaft superior to the lesser trochanter of the femur.