The calcar, also known as the calcaneum [1], 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 [1][2], this is to help spread the interfemoral membrane [3], 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 [3]. The oldest known ancestor to present day bats, Icaronycteris index, apparently did not have a calcar or spur as evidenced by fossil remains.[4].
Calcar (Femorale) also refers to the dense, vertically orientated bone present in the posteroemedial region of the femoral shaft under the lesser trochanter of the femur.