386 BRITISH FOSSIL REPTILES. 



vertebrae of birds, and doubtless admitted a production from the air-cells extending 

 along the neck of the Pterodactyle into the cancelli of the osseous tissue. The neural 

 arch rests upon the three anterior fourths of the centrum ; it expands as it passes back- 

 ward ; and there, also, as it rises, until it sends off from each posterior angle the zyga- 

 pophysis (PL 7, fig. 11, z), which has a tubercle above, and a flat articular surface 

 below, looking downward and a little outward and backward. The small tubercles 

 at the fore part of the neural arch, shown in fig. 13, to which the neurapophyses of 

 the atlas are ligamentously connected, may be the stunted homologues of anterior 

 zygapophyses. The neural spine begins by a low ridge between those tubercles, 

 increasing rapidly in thickness behind ; but it has not been preserved in its full 

 height in any specimen. 



In the small atlas and axis figured in PI. 8, figs. 1 — 4, the line of suture 

 between the bodies of these two vertebrae is distinct. In a somewhat larger specimen, 

 the centrum of the atlas was separable by a smart blow, and showed the true anterior 

 surface of that of the axis, as shown in PI. 7, fig. 13 ; this surface is very slightly con- 

 cave, with a submedian prominence. The neural canal expands at its posterior outlet. 



The small atlas and axis (PI. 8, figs. 1 — 4) not only differ in size from the 

 specimen (figs. 5 and 6), but also in the smaller relative size of the articular surface of 

 the zygapophysis, and the greater relative expansion of the back part of the centrum : 

 the specimen belongs to another species of Pterodactyle. On comparing the atlas and 

 axis of the Pterodactyle with that of the bird, the Ostrich for example, the atlas in the 

 bird is represented by the neurapophyses, which have coalesced below with a hypapo- 

 physis, forming an irregular ring of bone. The centrum has coalesced with that of 

 the axis, form.ing a small prominence, convex anteriorly, and filling up the vacuity at 

 the upper part of the cup excavated in the fore part of the hypapophysis ; the neur- 

 apophyses are broad in the bird, and overlap the anterior zygapophyses in the axis ; 

 they meet above the neural canal, but long retain the separating fissure there, in the 

 Ostrich ; the centrum of the axis is broader before than behind. A short process, like 

 a connate pleurapophysis, from the fore part of the centrum, unites with a diapo- 

 physis from the neural arch to form an arterial canal. The pneumatic foramen is 

 behind the diapophysis, and conducts to the cancellous tissue of the neural arch. The 

 centrum is produced into a strong hypapophysis below the posterior articular surface ; 

 but not expanded laterally into transverse processes, answering to parapophyses, in 

 the Pterodactyle. The hinder articular surface of the centrum of the axis in the 

 bird is convex transversely, but concave vertically, not simply convex, as in the 

 Pterodactyle ; thus a portion of the vertebra of that reptile, notwithstanding its 

 pneumatic structure, might be distinguished from the vertebra of a bird. 



In the ordinary neck-vertebrse of the Pterodactyle the centrum is oblong, subde- 

 pressed, sHghtly compressed at the middle, subcarinate (PL 8, figs. 11, 12, h), or 

 with a low obtuse hypapophysis (fig. 18) at the fore part of the under surface. 



