66 DENNIS, ON THE MICROSCOPIC 



lateral extension of their skin, which being spread out like a 

 parachute sustains them for some time. I have examined 

 the leg-bones of two species of flying Phalangers, and a few 

 words will be necessary about them. In such animals light- 

 ness and elasticity, but especially the former, are of para- 

 mount importance. In the smaller flying Phalanger, the 

 Haversian tvibes, especially in the tibia, are large and 

 numerous, the lacunse 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 gives the 

 appearance of the Haversian tubes in the tibia of the smaller 

 species ; they are disposed in the most advantageous manner, 

 running in parallel longitudinal courses, with only an occa- 

 sional junction between them. 



Amongst recent reptiles, the Draco volans is enabled, by 

 an extension of its false ribs, to expand a fan-like membrane, 

 by which it can, like the Phalangers, for a little time sustain 

 itself in the air. I have examined also the bones of this most 

 interesting reptile ; they are hollow and thin, but strong, 

 without Haversian tubes, and having very numerous lacunse ; 

 the canaliculi are fine for a reptile, but partake, in other 

 respects, of the reptilian characters. The ulna of the Draco 

 volans compares very much with that of the frog in the cha- 

 racter of its lacunar ; in the femur they approach more those 

 of the Phalanger ; there is, however, an admixture of roundish 

 lacmue, but this admixture is apparent also in the bones of bats. 

 As the animal is an agile climber, strength in its bones is 

 as requisite as lightness ; and as the parachute is altogether 

 disconnected from its limbs, it Avould seem to be with the in- 

 tention of giving them absolute freedom — the long ribs 

 being mobile, and the membrane folding up by the creature's 

 sides until occasion requires its expansion, ofteriug no im- 

 pediment in the way of its progression (fig. 4 represents the 

 ulna) . 



Before I make any remarks upon the Pterodactyle, I 

 would describe some of the leading features of the bones of 

 the gannct, and say something of the structure of biixls in 

 general. The gannet being peculiarly characterised hy the 

 length of its wing-bones, is a very appropriate bird to 

 compare both with the bats and Pterodactyles ; its habit is 

 to take its finny prey by falling headlong upon it with 

 immense velocity, and often from some height. It possesses 

 to a surprising extent the power of inflating its body with air, 

 and from the great buoyancy of its bodj would seem to be 

 iucapable of diving, and, though wcb-footcd like tlic cor- 



