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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



detail a little later. Here we need only note that a bird's front limbs 

 have been completely specialized for flight ; each wing forms a struc- 

 ture of peculiar beauty and complexity (pi. 4, fig. 1). Unlike that of 

 any other flying animal, the wing surface in a bird is made up of 

 feathers, all fitting together to form an efficient lifting surface and yet 

 capable of being neatly furled when not in use. There are two chief 

 kinds of gliding birds: the low gliders, such as shearwaters and al- 

 batrosses ; and the high soarers, such as eagles and vultures. 



Figure 3. — The flying squirrel (Glaucomys), showing its gliding flight with limbs 



outstretched. 



Of mammals, the best known gliders are the bats. A bat's wing 

 surface is probably not unlike that of a pterodactyl, but instead of 

 spreading from the fourth finger only, it is spread between the body 

 and all the fingers of the hand except the thumb. A bat's flight is not 

 very different from a bird's flight; but it differs in this, that bats 

 normally rest by day and fly by night. How they do this we shall see 

 later on. 



There are also a number of mammals capable of gliding from tree to 

 tree: flying phalangers, flying squarrels; all of these have membranes 

 of some kind running between their wrists, body, and back legs. When 

 such an animal is walking, the membrane is slack, but when it takes 

 to the air for a glide, the front and hind limbs are spreadeagled, 



