Archeeopteryx macrura. 4.49 
sternum takes a notable part therein by developing consider- 
able surfaces for the attachment of muscles. Sometimes it is 
a rounded buckler, as in Rhamphorhynchus, sometimes a keel, 
asin the Bats, or a combination of both, as in Birds, where the 
union of the three bones, coracoid, scapula, and furcula, forms 
an immovable pyramid, bearing the arm-joint on its summit. 
The humerus is a little elongated ; but it becomes very thick 
and presents powerful muscular ridges. The fore-arm is still 
more elongated. If one of the two bones of the fore-arm 
becomes rudimentary, as in the Bats, the other gains in 
length and thickness. In general, the anterior extremity in 
all flying animals is elongated altogether, which is in direct 
opposition to its conformation in animals that leap, such as 
the Dinosaurs, where it is shortened. If Birds’ wings seem 
to us short, it is because of their articulations bent in an ex- 
treme zigzag ; and we may easily convince ourselves, either by 
measuring or extending them, that, when extended, the wings 
reach the ground in nearly all Birds, except the long-legged 
Waders, if the body be put in the position which it takes in 
a quadruped. 
If all these conformations are common to all flying animals, 
differences begin to show themselves at the carpus. The 
animals that fly by means of an extended membrane preserve 
the primitive number of five digits, these digits being always 
elongated and slender; while in those that fly by means of 
feathers the number of digits is reduced by the union of some 
with others, either by symphysis or very strong igaments— 
carpus and metacarpus following these modifications of the 
digits. Both these methods of adaptation, so different, and 
even so opposite, thus depend on the nature of the integu- 
ments with which the surface opposed to the air is furnished. 
Let us first examine adaptation to flight by means ofan ex- 
tended membrane. 
In the Flying-squirrels (Pteromys), the Galeopithecus, and 
in some Marsupials (Petaurista) we see all the digits free and 
armed with claws. The skin, covered with hair, and extend- 
ing in folds between the fore- and hind-limbs, only serves as 
a parachute, and not as an active flying-membrane. — 
SER. IV.—VOL. IV. 2H 
