22 CHARACTERS OF BATS CHAP. 
brane. The mammae are thoracic; the placenta discoidal and 
deciduate. The cerebral hemispheres, which are smooth, do not 
extend over the cerebellum. 
This large order of mammals was once placed with the Primates. 
There is no doubt, however, that they form.a perfectly distinct 
order; no knowledge of fossil forms in any way bridges over 
the gap which distinguishes them from the highest mammals. 
The most salient feature in their organisation is clearly the 
wings. These consist of membrane, an expansion of the in- 
tegument, provided with nerves, blood-vessels, ete., which mainly 
lie stretched between the digits 2 to 5. These digits themselves, 
which are enormously elongated, act like the ribs of an umbrella, 
and when the wing is folded they come into contact. Besides 
Fig. 254.—Barbastelle. Synotus barbastellus. x4. (After Vogt and Specht.) 
this part of the flying apparatus there is a tract of membrane 
lying in front of the arm, which corresponds to the wing mem- 
brane of the bird, but which in the Bats takes quite a subordi- 
nate place. In the bird, on the other hand, there is a metapa- 
tagium, which is the main part of the wing of the Bat. It 
seems just possible that in Archaeopteryx the metapatagium was 
more Bat-like. Furthermore, a steering membrane, like that 
which fringes the tail in some Pterosaurians, lies interfemorally 
in Bats, and includes the whole or a part of the tail. The pollex 
takes no share in the wing, but projects, strongly armed with a 
claw, from the upper margin. 
The bones of this order of mammals are slender and marrowy ; 
they are thus light, and subserve the function of flight. A 
most remarkable feature among the external characters of the Bat 
tribe is the extraordinary and often highly complicated mem- 
branes which surround the nostrils. These are at least often 
