554 CHORDATA. 
Thus the fox-bat has a dental formula of 2-3-3-3, a 
considerable reduction in number from that of the typical 
eutherian. The number agrees closely with the fruit-eating 
or frugivorous Primates, the marmosets having 3-4 3-4. 
The cervical vertebree are small and compressed and 
carry very small neural spines. The thoraco-lumbar ver- 
tebree, bearing fourteen pairs of ribs, are seventeen in 
number. They are all set in one curve, have few processes, 
very little motion on each other, and are not infrequently, 
as in birds, largely ankylosed or fused together. In each 
case rigidity of the central axis is a necessity. 
The caudal and sacral vertebre are fused together. 
The thoracic cavity is spacious and the ribs are compact. 
The sternum has a prominent median keel, which is largest 
on the presternum but is continued as a series of smaller 
keels on each sternebra. The scapula is large and trian- 
gular and is firmly connected with the presternum by the 
clavicles. These are stout and curved though not shortened, 
as in the mole. The fore-limb is enormously long and the 
bones are slender. The ulna is vestigial, like that of the 
horse, but the radius is very long. It bears six small carpal 
bones and five digits. The pollex is short and free from 
the wing’; it bears a claw. The other four digits are enor- 
mously elongated and serve when separated to extend the 
wing-membrane. The second digit terminates in a claw, 
but the others end in tapering phalanges. 
It is instructive to compare this wing with that of the bird. We 
see at once that the same ends are attained by a different method. 
The main axis of support is the fore-limb in each case, supplemented 
in the birds by the reduced digits. The lateral axes are formed in 
the bird by the shafts of true feathers and in the bat by the digits. 
Lastly, the vanes of the feathers serve the same mechanical purpose as 
the patagium of the bat. The sternal carina is found in each, as 
an attachment for the pectoral or ‘‘ flight” muscles, but whereas the 
fulcrum of the fore-limb is attached to the sternum mainly by the 
coracoid, supplemented by the clavicle in the bird—the bat having, 
as a mammal, practically lost its coracoid in early times, has to rely 
upon the clavicle alone. 
We may recollect that the mole has a keeled sternum 
and a strong bony junction of scapula to presternum. 
These are alike due to hypertrophy of the pectoral muscles, 
in its turn connected with excessive use of the fore-limb, 
