20 



THE GEOLOGIST. 



its false ribs, to expand a fan-like membrane, by which it can ... for a little 

 time sustain itself in the air. . . . The bones of this most interesting reptile 

 are hollow and thin, but strong, without Haversian tubes, and having nu- 

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

 respects of the reptilian characters. . . . The gannet, being peculiarly cha- 

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

 compare both with the bats and the Pterodactyles ; . . . the humerus, 

 radius, and ulna partake of the (microscopic) characters observed in all 

 birds whose wings are long and pointed. In the humerus vertical section 

 (fig. 17) the Haversian tubes do not reticulate, but run nearly parallel, and 

 ultimately converging to a point, from which extends another tube, which 

 converges in a similar manner. . . . In the transverse section the Haversian 

 tubes appear as round dots, the lacunas as small irregular specks, and the 

 canaliculi beautifully reticulate, etc. . . . Furcula, fig. 18, coracoid, fig. 19, 

 femur, fig. 20 ; transverse section, fig. 21 ; tibia, fig. 22, rib, fig. 24 ; furcula 

 of swift, fig. 6. . . . The structure of the gannet ... in the principal bones 

 admirably exhibits the beautiful adaptation of the microscopic structure of 

 bone to the movements, habits, and well-being of the creature ; and in no 

 bone is that adaptation more closely shovrn than in the coracoid — the cir- 

 cular reticulations of the Haversian canals in which bone are assuredly 

 designed to enable it to sustain the shock it must receive when the bird 

 impinges on the water. . . . 



" For ordinary purposes, the gelatinous homogeneous bones of fishes seem 

 sufficiently strong for their mode of living, but a much more elaborate 

 structure is requisite in the higher vertebrata. ... In the Ichthyosaurus 

 and JPlesiosaurus, the canaliculi, though finer than those of some other 

 saurians, are few in number ; hence their bones are perfectly distinct from 

 those of mammifers or birds." 



Mr. Dennis then passes on to the more direct consideration of the 

 structure of birds' bones : — 



" Comparisons," he says, " with the same bones in birds of similar flight 

 and configuration of the wing are highly useful in elucidating this subject. 

 Take, for instance, such birds as the ring- dotterel ; turnstone, dunlin, 

 pigmy curlew, little stint, and other birds of that description, whose mode 

 of flight is so similar that . . . the most practised sportsman ' cannot - dis- 

 tinguish one from the other by its flight. If we examine a portion of the 

 ulna taken from the .same part of the Done in any one or all of these birds, 

 we shall at once observe a similar and singular correspondence in the dis- 

 position of the Haversian tubes. Examine next the ulna of the greensand 

 piper, a bird whose wing is broader . . . and not so pointed, . . . and 

 whose flight is easily recognized from its congeners, . . . and there is ' an 

 entirely different arrangement of the Haversian tubes, which are reticulated 

 in every direction, while in the rest they observe longitudinal directions." 



What conclusion can we arrive at, Mr. Dennis asks, but that these 

 tubes are arranged in accordance with the flight of birds ? and in 

 confirmation of this view he points out that the starling, raven, jay, 

 etc., in which the secondary quills are well developed, have all fine 

 and numerous reticulated tubes : — 



" In the fowl they are very powerful, and the ulna contains numerous 

 and line tubes ; in the owls the same ; in the hawks the Haversian tubes are • 

 large and much reticulated, and are easily recognized from those of other 



