Patagium

The patagium is a membranous structure that assists an animal in gliding or flight. The structure is found in bats, birds, some dromaeosaurs, pterosaurs, and gliding mammals.

In bats, the skin forming the surface of the wing is an extension of the skin of the abdomen that runs to the tip of each digit, uniting the forelimb with the body.

The patagium of a bat has four distinct parts:

In the flying pterosaurs, the patagium is also composed of the skin forming the surface of the wing. In these ornithodirans, the skin was extended to the tip of the elongated fourth finger of each hand.

The patagium of a pterosaur had three distinct parts:

Flying squirrels, sugar gliders, colugos, anomalures and other mammals also have patagia that extends between the limbs; as in bats and pterosaurs, they also possess propatagia and uropatagia. Though the forelimb is not as specialised as in true flyers, the membrane tends to be an equally complex organ, composed of various muscle groups and fibers. Various species have styliform bones to support the membranes, either on the elbow (colugos, anomalures, Greater glider, Eomys) or on the wrist (flying squirrels).

In gliding species, such as some lizards and flying frogs, it is the flat parachute-like extension of skin that catches the air, which allows gliding flight. These to be structurally less complex than

...
Wikipedia