554 CHORD AT A. 



Thus the fox-bat has a dental formula of f-x-§-|, 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 f-^ f-|. 



The cervical vertebrae are small and compressed and 

 carry very small neural spines. The thoracolumbar ver- 

 tebrae, 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 vertebrae 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-Hmb 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, 



