CRETACEOUS FORMATIONS. 101 



gradually decreases to a diameter of nine lines. The side imbedded in the chalk is 

 convex ; that exposed to view is nearly flat ; but it is somewhat crushed ; the longer 

 portion of the other bone is also too much crushed to give an idea of its natural shape. 

 Like the portions of bone in T. XXIV, these also present a thin wall of compact bone 

 encompassing a very wide medullary or pneumatic cavity ; the thickness of the wall 

 equals that of the same part of the ulna of the Pelican, T. XXXII, fig 1. 



In the long bone, fig. 4, T. XXX, the original of the fig. 1, PI. 39, of the 

 ' Geological Transactions,' 2d series, vol. vi, the natural shape of the bone is better 

 preserved ; but, unfortunately, only one small portion of the articular surface is 

 preserved at the expanded end, and this merely exhibits part of a shallow concavity, 

 with a thin well-defined border, fig. 4*, «. From this articular end to the opposite 

 fractured end of the shaft, the bone measures twelve inches. The breadth of the 

 expanded end is one inch and a half, whence the shaft gradually diminishes to a 

 diameter of nine lines at its middle part, and more gradually increases to a diameter 

 of eleven lines at the broken end. 



The bone is very slightly bent lengthwise at its expanded end ; it is straight in 

 the rest of its extent ; its shaft is unequally three-sided, with the sides smooth and 

 flat, and the angles rounded off. The compact osseous wall is about the tliird of a line 

 in thickness, and incloses, as in the other specimens, an uninterrupted wide cavity. 

 One of the sides of the bone equals the extreme breadth of the shaft; a second 

 measures seven lines across, the third five lines ; the second side increases in breadth, 

 at the expanded end, in a much greater degree than the third or narrowest side ; and 

 this seems to have been indented by a natural fossa, and to have been perforated, at^, 

 for the admission of air to the cavity, before terminating at the border of the articular 

 concavity. The true nature of this perforation, which I formerly apprehended might be 

 accidental in the fractured state of that end of the bone, and before the discovery of 

 other specimens, is illustrated by the presence of a similar perforation in the larger 

 sized corresponding bone fig. I, p; and gives additional evidence of the remarkable 

 fact of the agreement of the flying-reptUes with birds in the extension of the air-cells 

 into the cavities of the bones. 



Tab. XXIV, fig. 2, is the terminal portion of a long bone, with the articular end 

 again unfortunately destroyed, so as to deprive us of one of the best guides to the 

 determination of the fragment. So much of it as is preserved corresponds pretty 

 closely with the proximal end of the foregoing bone : it is subtriedral, with the angles 

 rounded off ; the broadest side is imbedded in the chalk ; the expansion of the exposed 

 surface is chiefly due to that of the next broadest side ; and the narrowest side, as it 

 approaches the articular end, is impressed by a deep and narrow fossa, in which there 

 is an interruption of the thin walls of the bone in the corresponding position of that, 

 which, in the foregoing specimens, I have called a "foramen pneumaticura." A portion 

 of the bone indicates the extension of a process beyond the articular cavity, which 



