Vol. 69.] SKELETON OF OBNITHODESMUS LATIDENS. 387 



The Radius. 



The radius (PI. XXXIX, figs. 5 & 6,' R.) is slightly shorter than 

 the ulna, and very much less in diameter throughout its length. 

 Prosimally, it is ventral to the ulna, and placed on the preaxial side. 

 From here it gradually rises and crosses over the shaft of the ulna, 

 until the distal end is entirely dorsal to it, and occupies the central 

 half of the ulna, the remaining quartei-s of that bone being visible 

 on each side. The proximal articulatory surfaces fit the trochlea 

 on the preaxial ventral border of the distal end of the humerus. 

 Proximally, the radius is compressed dorso- ventral! y and-expanded 

 pre-postaxially. Its dorsal surface is convex, while the ventral is 

 concave, and convex on its pre-postaxial border. The last-named 

 quickly becomes an angular ridge, which is continued distally for a 

 short distance ; then the whole bone becomes circular and rod- 

 like, until it approaches the distal end, where it gradually expands 

 pre-postaxially. The distal articular end is a simple convex pulley. 

 On the postaxial side of the dorsal surface of the distal end there 

 is a well-developed longitudinal rid^e, and the stria' made by the 

 fibres of a muscle traversing the bone from the preaxial border are 

 visible. On the ventral surface contiguous to the postaxial border 

 there is a longitudinal groove. 



The Ulna. 



The ulna (PL XXXIX, figs. 5 & 6, U.) is a very powerful bone. 

 In the median area of the shaft it has a diameter three times greater 

 than that of the radius. The shaft is straight, but an expansion 

 of both articular ends gives a somewhat curved outline to the pre- 

 postaxial borders. The proximal end (PI. XXXIX, fig. 4) is 

 roughly triangular, the apical side being on the dorsal surface. 

 There is no olecranon. On the preaxial side of the articular 

 surface is a triangular area, in extent equal to a third of the 

 articulation : this is weakly convex, obliquely placed, and looks 

 ventrally. The remaining two-thirds of the articulation constitute 

 a platform, which is raised above the other third and looks distally. 

 In the centre of this space is a high and strong V-shaped ridge 

 (PL XXXIX, fig. 4, E.) with the angle directed postaxially. The 

 ventral branch articulates in the valley (PL XXXIX, fig. 3, v.) 

 on the distal end of the humerus. Thus the overlapping dorsal 

 edge of the humerus enters between the two branches, where they 

 converge at the augle. On the dorsal side of the angle of the V 

 is a shallow concavity, in which the tubercle of the ulnar condyle 

 of the humerus rests, and acts as a stop to any dislocation in a 

 preaxial direction, as the angle of the V does in the opposite. 

 Along the postaxial half of the V, and ventral to it, is a curved 

 groove, in which the convex ventral border of the postaxial side of 

 the humerus articulates. A pneumatic foramen is present in the 

 centre of the ventral surface of the shaft, near the articulation. 

 Distal to this is a high robust ridge (PL XXXIX, fig. C, ri.\ 



