Vol. §9.] SKELETON OE ORNITHODESMUS LATIDENS. 407 



other genera. Probably it will be found that the decussation of the 

 ulna by the radius is not peculiar to Ornithodesmus. It certainly 

 occurs among the Cambridge Greensand specimens. In the distal 

 ends of the radius and ulna of Pterodactylus compressirostris from 

 the Chalk Pit, Burham (Kent), which have been figured by Owen, 1 

 the radius is seen decussating the ulna. On the first plate the 

 ventral, and on the second the dorsal, surfaces of both bones are 

 exhibited. Seeley ~ has called attention to the fact that the fossil 

 in fig. 1, pi. xxiv of Owen's ' Cretaceous Peptilia ' is ' figured for 

 the humerus ' and, further, that ' the less well-preserved bone in 

 that figure exhibits the ulna in its true position behind the radius' : 

 this, I think, should read, ' the radius in its true position behind 

 the ulna.' In view of the similarity of the humeri from this chalk- 

 pit, there cannot be much doubt that they belonged to the genus 

 Ornithodesmus. In the reconstruction of the hand of Rhampho- 

 rhynclius Jcokeni by Dr. Plieninger, the distal end of the radius is 

 partly behind the ulna, but in all other figures of restorations the 

 radius is placed at its distal end parallel with the ulna. These 

 reconstructions have been made from specimens in which the 

 bones are compressed and displaced. 



The fact that proximally the radius is in front of (ventral to) 

 the ulna has long been known. As the distal end of the radius 

 gradually worked into a dorsal position, either the proximal carpal 

 expanded dorsally for the new articulation (the ulna by expansion 

 at the distal end taking the place of the former radial articular 

 surface), or at one period the radius articulated with a separate 

 carpal, which, under the same influence, followed the radius, and 

 became fused on the original dorsal surface of what is now the one 

 proximal carpal bone. The latter, I think, was the case. 



The radius and ulna are not separated in the central region of 

 their shafts, as in birds. 



The Ulna. 



The ulna is more reduced in the median region of the shaft, 

 more expanded at the extremities, and has more highly-specialized 

 articulations than in any other known example. The proximal arti- 

 culation is far removed from the trochlear joints of the European 

 and American specimens ; but some of the Cambridge Greensand 

 specimens included in the genus Ornithocheirus exhibit it, although 

 either in an incipient or in a degraded stage. 



The Pteroid Bone. 



Dr. Plieninger 3 says that, in the long-tailed forms, the pteroid 

 is a short compressed rod, in the short-tailed forms slender and 

 thin. Ornithodesmus possessed the type of the long-tailed forms. 



1 ' Foss. Eept. Cret. Form.' 1851-64 (Monogr. Pal. Soc.) pi. xxiv, figs. 1-2 & 

 pi. xxx, fig. 5. 



2 'The Ornithosauria ' 1870, p. 45. 



3 'Pterosaur, d. Jura Schwabens' Palteontographica, vol. liii (1907) p. 308. 

 Q. J. G. S. No. 274. 2 e 



