368 



BRANCH CHORDATA 



Geologic Distribution. — In North America they date back to 

 the Eocene Period. The Miocene Palceoerina'ceus differs so 

 little from the existing genus, Erina'ceus, that the latter may be 

 called " one of the oldest living genera of mammals." 



Use to Man. — Many mammals of this order have fine soft 

 fur. They are also of value as insect destroyers. A single mole 

 is said to d('V(jur twenty thousand insects annually. 



Order X. Chiroptera. — The bats have many points of resem- 

 blance and structure with both lemurs and insectivores. But 

 they are distinguishable from all other mammals by their power 



Fig. 295. — Skeleton of picropus: iS^, JSlcrnuin; (7, clavicle; Sc, scapula; 

 H, humerus; R, radius; U, ulna; D, thumb; J I, ilium; P, pubis; Js, 

 ischium; Fc, femur; T, tibia; F, fibiila. (After Owen.) 



of zigzag flight. " The wings " (Fig. 295) consist of an integu- 

 mental membrane, supported by the digits (two to five), which 

 are greatly elongated and are folded together like the ribs of an 

 umbrella when the wings are folded. A membrane between the 

 hind legs and tail, when present, is used for steering. 



The expanse of wing, compared with that of the body, is 

 greater than that of most birds, but the muscles are weaker. 

 The first digit, or thumb, is short, free, and strongly clawed, 

 " and sometim(>s a sucker, by which bats scramble about rocks 

 and trees, recalls the similar organ in that primitive lizard-like 



