72 DENNIS, ON THE MICROSCOPIC 



instruments of fligHt, as they seldom take wing, but inva- 

 rialjly dive, when they are approached, and may be observed 

 to use their wings under the water with siu'prising ease and 

 agility. The Aving, in fact, is more constructed like an oar, 

 and the bones are flat^ especially the humerus, and the wrist 

 is strongly braced to the forearm. The ulna and femur of 

 the red-throated diver exhibit a very singular sort of sinuses 

 in the Haversian tubes, which seem to swell out ; the struc- 

 ture of these is peculiar, and at present I have not been able 

 quite to make it out, there being a great deal of oily matter 

 in the bones of these birds. ^ I have observed a structure of 

 bone something similar in the medullary ca^dty of the ulna 

 of the Draco volans (fig. 4). The tibia of the red-throated 

 diver (fig, 5) is introduced on account of this peculiarity. 

 The goosander exhibits somewhat similar reticulations to 

 the Stonesfield fossil. 



Having so far given the general features of the microscopic 

 characters of the bones of birds, we may now attempt to 

 grapple with the microscopic structure of that most singular 

 reptile the pterodactyle. 



The Pterodactylus longirostris perhaps affords us the most 

 perfect means of studying the singular proportions of its 

 skeleton. A larger and less-perfect, but exceedingly useful 

 one, Avas discovered by Miss Anning, at Lyme Regis, and 

 which is now placed in the British Museum. Also portions of 

 the jaws of a very large kind have been discovered in our 

 Chalk formation, with other bones now supposed to have 

 belonged to a similar animal. From these specimens we 

 learn that the animal was a true saurian, apparently adapted 

 for flight, and for arboreal and terrestrial movements, and 

 instead of possessing, like the bat, an extension of all the 

 fingers, it had only one prolonged, the others being used for 

 progression ; it differed also from the bat, in having a well- 

 developed ulna as well as a radius. There did not, however,, 

 exist between them the space, that, in birds, is adapted to 

 receive so much muscular power; but as the forearm per- 

 formed double the duty of that of the bat, we may reasonably 

 suppose that its muscles were more numerous. In the bat, 

 the extensor of the fingers is a small muscle, which, arising 

 from the external condyle of the humerus, and, passing over 



* I find ill the lower jaw of the Pleropiis numerous Haversian tubes, 

 parallel with the Iciiglh of the jaw, and having enlargements or sinuses in 

 them very similar to those in the red-tlii-oated diver. Surely this is a fur- 

 ther proof of tiie adaptation of the Haversian tubes to the habits of the 

 animal. Tiie tubes are also branched and conneeted, very different to the 

 way iu which they appear in other parts of the skeleton. 



