Icaronycteris is an extinct genus of microchiropteran (echolocating) bat that lived in the early Eocene, approximately 52.2 million years ago. Four exceptionally preserved specimens are known from the Green River Formation of North America. There is only one thoroughly described species of bat in the genus, I. index,  although fragmentary material from France has also been tentatively placed within Icaronycteris as the second species I. menui.
Icaronycteris measured about 14 centimetres (5.5 in) long and had a wingspan of 37 centimetres (15 in). It closely resembled modern bats, but had some primitive traits. The tail was much longer and not connected to the hind legs with a skin membrane, the first wing finger bore a claw and the body was more flexible. Similarly, it had a full set of relatively unspecialised teeth, similar to those of a modern shrew. Its anatomy suggests that, like modern bats, Icaronycteris slept while hanging upside down, holding onto a tree branch or stone ridge with its hind legs.
Barry Cox, Colin Harrison, R.J.G. Savage, and Brian Gardiner. (1999): The Simon & Schuster Encyclopedia of Dinosaurs and Prehistoric Creatures: A Visual Who's Who of Prehistoric Life. Simon & Schuster.