STRUCTURE OF FOSSIL BONE. 65 



exceedingly insignificent when compared with those of birds ; 

 their bones are harder in texture, and only the very large 

 bats possess Haversian tubes, for even the largest indigenous 

 to this country are without them,"^ and the Pterojius, the 

 largest of the family, has only short and straight ones, and 

 these not at all numerous. Not carrying elastic and power- 

 ful quills, the bat does not need the same muscular develop- 

 ment that is necessary for the bird, a simpler power of 

 extension and flexion of the wing is all it requires in its 

 forearm ; where great muscular power is wanted there it has 

 it. The chest is enormously developed, the ribs are strong 

 and flat, the sternum has a ridge, the scapulae are approxi- 

 mated to those of the bird, and the whole strength of the 

 animal is concentrated in the chest and shoulders. In fig. 1, 

 plate VI. are shown the Haversian tubes and lacunae of 

 the Pteropus, or flying fox as it is popularly called ; the Haver- 

 sian tubes are more like long lacunae, and are, in the humerus, 

 straight, unconnected, and not numerous, but the lacunae 

 are very abundant; these become more fusiform in the 

 phalanges, as may be seen in fig. 2, which represents a 

 phalanx with its medullary cavity of one of our small bats. 

 The canaliculi of the bat are large for mammals. The 

 Haversian tubes in the phalanges of the Pteropus are longer 

 and finer than in the humerus, and a slight ramification is 

 seen in the extreme ones. I conceive the reason of the 

 paucity or absence of these tubes 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 their great length and slenderness would 

 make the wing so pliable, that it would be powerless in 

 resisting the air, and thereby enabling the animal to sustain 

 itself in flight. Lightness is obtained by numerous lacunae, 

 and numerous and comparatively thick canaliculi, and no 

 more elasticity is imparted to the bone than is sufficient to 

 preserve its structure, and obviate any fracture, in flight. In 

 the bird, on the contrary, elastic resistance to muscular pres- 

 sure is of much importance ; and hence appears to have 

 arisen that admirable application and adjustment of the 

 Haversian tubes, so conspicuous in the economy of birds. 

 Besides bats, there are other mammifers, such as the Galeo- 

 pithecus, flying Phalangers, &c., which, though they do not 

 fly in the true acceptation of the word, yet are enabled to 

 perform flights to some extent in the aii' by the aid of a 



* I have found them in i\\&jaio,& thick stem with a few straight brandies 

 between the fangs of tlie teeth. Tlie jaw is a good object of comparison wil li 

 that of the mole. (^Sne p. 7n.) 



