I was fortunate recently to acquire a copy of Franz Marc, the 1936 biography by Alois J. Schardt. This is a cool book with a lot of drawings I had not seen before, including this study. This bat is a lot more cheerful than the one in Hoffnungslos. The echolocation abilities of bats were not identified until 1938 (but bats flew first, 53 million years ago, and then developed this type of sonar also used by dolphins and moles), so Marc wouldn’t have known about it. It is really cool to see that Marc realized how amazing bats are, though.
Anatomischestudien 1907 detail of bat, from Franz Marc