MA.CKIE — ON FOSSIL BIRDS. 
19 
elucidation of the question) into the consideration of the structure, 
not only of birds but of both mammals and reptiles also, the latter, 
bj the Pterodactvles, being intimately connected witli the subject. 
*' Amongst mammifers, the bats alone have the poAver of maintaining a 
continued flight, by the rapid vibrations of membranous wings, which are 
in fact but a modification of the arm and hand, so adapted as to enable 
them to extend a thin canvas by which their bodies are drawn through 
the air. To this end the fingers are greatly increased in length as well as the 
humerus and radius ; the scapula also is large, and lengthened in a slanting 
direction at its lower portion for the better attachment of muscular power ; 
and to afford greater support to the stroke of the wing ; the clavicles are 
curved like the furcula of the bird, and perform the united functions of 
the furcula and coracoids, preventing the compression of the chest, and 
keeping the humerus in its place when acted upon by the pectoral muscles. 
. . . But there is no deviation in this flying mammal from the plan nature 
has laid down for the construction of mammifers ; the bat, because it flies, 
is not a bird, no more than the bird that swims and cannot fly is a fish. . . . 
The wing of the bat is in accordance with the same design that framed the 
hand of man. . . . The bones of bats are excessively strong, hard, and 
semitransparent. . . . The muscles of the fore arm are extremely insigni- 
ficant when compared with those of birds ; their bones are harder in texture, 
and only the very large bats possess Haversian tubes, even the largest 
indigenous to this country are without them ; * and the Pteropus, the 
largest of the Aimily, has only short and straight ones, and these not at all 
numerous. ... In fig. 1, pi. vi. arc shown the Haversian tubes and lacunae 
of the Pteropus or flying fox, as it is popularl}' called ; the Haversian 
tubes are more like long lacuna?, and are in the humerus straight, uncon- 
nected, and not numerous, but the lacuna) are very abundant; these be- 
come more fusiform in the phalanges (fig. 2). . . . The canaliculi of the bat 
are large for mammals. ... I conceive the reason of the paucity or absence 
of these tubes (the Haversian) in the bats is to be found in the character 
of their wings, and the habits of the animal. Bats require light but strong 
bones, hardly at all flexible, for were they so the great length and slender- 
ness would make the wing so pliable that it would be powerless in resist- 
ing the air, and thereby enabling the animal to sustain itself in flight. Light- 
ness is obtained by numerous lacuna?, and numerous and comparativel}'" 
thick canaliculi, and no more elasticity is imparted to the bone than is 
sufiicient to preserve its structure, and obviate any fracture in flight. In 
the bird, on the contrary, elastic resistance to muscular pressure is of 
much importance ; and hence appears to have arisen that admirable ap- 
plication and adjustment of the Haversian tubes so conspicuous in the 
economy of birds. . . . Besides bats there are other mammifers, such as the 
Galeopithecus, flying Phalangcrs, etc., which . . . are enabled to perform 
flights to some extent by the aid of a lateral extension of their skin, which 
being spread out like a parachute sustains them for some time. ... In the 
smaller flying Phalanger the Haversian tubes, especially in the tibia, are 
large and numerous, the lacuna' are very numerous, and the canaliculi 
large; every particle of unnecessary weight seems to have been abstracted 
from their bones, that the light animal may float almost like a feather in 
the air (fig. 3). . . . 
" Amongst recent reptiles, the Draco volans is enabled, by an extension of 
III a footnote Mr. Dennis states that lie has "found them in the jaw, a thick ^tnn 
with a few straiglit blanches between the fangs of the teeth." 
