18 FOSSIL REPTILIA OF THE 



the outer ridge, is rather more produced and of a less regular curve than the inner ridge. 

 The outer ridge, a, begins by a rising at the middle of the fore part of the distal end of 

 the shaft, which bends obliquely outward and meets the outer angle of that part of 

 the shaft where the outer trochlear ridge begins to be prominent ; this ridge then 

 extends with a feeble convex curve to the back part of the trochlea, where the con- 

 vexity of ihe curve increases, and it terminates by projecting a little beyond the level 

 of the outer almost flattened side of the trochlea (fig. 10). The articular surface, as it 

 extends from the margin of this element of the trochlea inward, is first gently convex, 

 then sinks to a concave channel by the side of the low median convexity. The inner 

 ridge 6, begins from the inner side of the fore part of the bone, and describes a pretty 

 regular semicircular curve as it extends backward and a little outward, to terminate 

 near the middle of the back part of the distal end of the shaft ; thus owing to the 

 termination of the inner ridge near the middle of the back part, and to the 

 beginning of the outer ridge near the middle of the fore part, of the metacarpal bone, 

 these principal ridges of the trochlear joint recede from each other at the middle of 

 the joint, and approximate at the fore and back ends of the joint. As the back ends 

 of the two lateral ridges are on the same transverse line, and the front end of the 

 inner ridge rises higher upon the shaft than that of the outer ridge, this is by so much 

 the shorter of the two. The low middle ridge c, is much shorter than either of the 

 lateral ones, being confined to the lower and middle part of the trochlea, to which it 

 gives an undulating transverse outUue (fig. 11). 



The figure of the metarcarpal bone of the wing-fiager, iu Pterodactylus 

 suevicus, Qnstd., does not show any trace of the mid-rising of the distal trochlear 

 joint. The back surface of that of the left wing shows a wide and moderately deep 

 excavation along the upper three fourths of the shaft. A portion of a similarly 

 shaped shaft of a long bone, in size matching that of the trochlear extremity (fig. 10), 

 is represented in Tab. IV, figs. 4 and 5. Although both ends are broken away, 

 yet the degree of expansion toward the upper end shows that this was not very far 

 from the proximal articulation. Tiie shaft is three-sided; two of the sides are nearly 

 flat or very feebly convex ; they meet anteriorly at an acute angle, but this is 

 rounded ofl" as shown in the transverse sections of figs. 4 and 5 ; the third and 

 shorter side is concave in the degree shown in the same sections. The lower of these 

 (fig. 5), indicates the extreme thinness of the compact wall of the bone, and th'C size 

 of the cancelli occupying that part of the shaft. 



The portions of the wing-bones of the Pterodactyles of the Cambridge Green- 

 sand, here described and figured, show^ the same superior proportions over those of 

 the great Pterodactyles from the Kentish Chalk, described and figured in a former 

 Monograph, 4to., 1851, as do the portions of jaw bones and teeth. 



The long diameter of the largest of the wing-bones, figured in Tab. XXX, fig. 1 , 



