GENERAL CHARACTERISTICS. 



97 



to hold fast and transfix the prey. The milk 

 dentition is very different from the permanent. 

 The teeth are furnished with backwardly 

 directed hooked points. But we must not 

 here try to enter on details. 



The ear is always large, the flap mostly 

 naked, and frequently, as in our long-eared 

 bat (Plecotus auritus), developed to such an 

 extent as to equal in length one-third of the 

 body, and to be capable of being rolled up 

 as in the galagos. The inner ear-covering 

 (tragus) is always very large, often tongue- 

 or spoon-shaped, and on it, as on the external 

 flap, there are always borders and projections 

 on the inner side which afford constant 

 characters to distinguish species. 



The structure of the wings deserves a more 

 thorough examination. The breast-bone or 

 sternum, to which the strong muscles for 

 effecting the downward stroke of the wings 

 are attached, is large, and is furnished with 

 a broad pre-sternum or manubrium, and a 

 long vertical longitudinal keel, which reminds 

 us of the structure seen in birds. The collar- 

 bones, which afford a support for the joint of 

 the fore -limb, are large and strong ; the 

 humerus, or bone of the upper arm, is the 

 largest and strongest bone in the whole body. 

 The fore-arm is likewise very much elongated, 

 but consists solely of the ulna, the radius being 

 rudimentary. The wrist is very short, and 

 is formed out of only a few small bones; all 

 the five bones of the middle hand, or what 

 anatomists call the metacarpal region, are 

 present, but that of the first digit is short and 

 thick, those of all the others long and thin. 

 The first digit remains more or less free, is 

 short and thick, and carries a large sharp 

 sickle-shaped claw. The other digits are 

 mostly without nails, and usually composed 

 of two remarkably long and thin joints (pha- 

 langes). In most of the flying-foxes (Pteropus) 

 the second digit is usually armed with a small 

 claw. " By means of these digits the wings 

 are spread out or folded up, just as the cover 

 of an umbrella is by means of. the ribs. The 



membrane forming the wings begins as a 

 fold of the skin at the neck, envelops the 

 fore-limbs and their digits, and, getting 

 attached to the sides of the body, is continued 

 backwards to the hind-limbs, which, for the 

 most part, it includes down to the ankle- 

 bones, and then passes to the tail, which is 

 sometimes left entirely free, sometimes just 

 fringed with the membrane, and sometimes 

 entirely inclosed by it. 



This membrane, apart from that portion of 

 it which is comprised between the elongated 

 digits, repeats the structure of such parachutes 

 as are met with in the flying-lemur, the flying- 

 squirrel, and the marsupials, all outgrowths 

 from lateral folds which occur in many em- 

 bryos. The portion belonging to the hand, 

 moreover, is likewise nothing else than the 

 excessive development of a primitive em- 

 bryonic condition. In all mammals hands 

 and feet are at first little oar-shaped processes, 

 within which the growing fingers lie embedded 

 in the tissue, and connected to their ex- 

 tremities by skin. In mammals possessing 

 fins or flippers (seals, whales, &c.) this em- 

 bryonic condition persists, but in most mam- 

 mals the digits grow out beyond the connect- 

 ing skin, which, however, often remains to a 

 greater or less extent as a web between the 

 toes (otters, beavers). In the bats the digits 

 grow to an extraordinary length, and carry 

 along with them the connecting membrane, 

 which thus persists as part of an organ of 

 flight. The wing of the bats is thus a spe- 

 cialized form of the fin or the embryonic limb, 

 and although it is a powerful organ of flight, 

 yet it has no resemblance in structure to the 

 bird's wing, which has arisen from quite 

 different modifications. 



The rapidity and ease of flight depend 

 upon the length and sharpness of the wings, 

 and this latter property is determined by the 

 proportion of the fifth to the third or middle 

 digit. Among our native bats the noctule 

 ( Vesperitgo noctula) and the long-winged bat 

 (Miniopteriis Schreiberi) are in no way in- 



