98 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 17, 



On the metatarsal hypothesis, the prominent convex articular sur- 

 face b, fig. 1, could l3e no other than part of the projecting trochlea 

 for the middle toe; but it is uniformly convex, not grooved as in 

 that trochlea in the Heron and other birds ; and, moreover, there 

 is no cleft in the fossil bone separating it from the adjoining con- 

 dyle, which cleft must have been present if the fossil had been a 

 tarso metatarsal. There is a second smaller rough elliptical surface 

 at c (fig. 1), on the concave side of the bone near the margin oppo- 

 site to that where the larger surface o is situated : the median mar- 

 gin of the smaller surface c is slightly raised. This surface is want- 

 ing in the tarso-metatarsals of the Heron and other birds. But 

 the most decisive evidence against the metatarsal character of the 

 fossil in question, now that the true position of the distal articula- 

 tion is determined, is given by the configuration of the opposite 

 or convex side of the expanded end of the bone, which proves that 

 end to have been simple, not trifid (fig. 2). 



After the recognition of the above characters of the Wealden 

 fossil, I proceeded to compare it with other bones of the skeleton 

 of birds, and found in the lower or distal extremity of the humerus, 

 the part that most nearly resembled the fossil in its present condi- 

 tion. This part presented, for example, the same unequal expan- 

 sion, the same smooth and gently undulating convexity on the back 

 part, and a similar concavity on the fore part ; a portion of the 

 distal articular surface corresponding with that marked a, and a 

 prominence of the outer condyle, which corresponds with that 

 marked b in fig. 1. In the humerus of most birds of flight, there is 

 a rough surface on the concave (anterior) side of the distal end, 

 about as far from the condyle or articular surface as the spot o in 

 the Wealden fossil, and there is also, in some, as the Cranes and 

 Herons, a small elliptical surface, lower down, near the opposite 

 margin, with the edge next the middle of the bone slightly raised, 

 like that marked c in fig. 1, but nearer the distal end. 



I am led by these resemblances to regard the fossil as the distal 

 end of a humerus, and by the analogy of that bone in Birds, to be 

 part of the left humerus. The thin compact walls (figs. 4 and 7) 

 and large cavity of the bone determine it to have belonged to a 

 creature endowed with the power of flight. Yet, although the first 

 bone of the wing in Birds offers the nearest general resemblance to 

 the fossil, there are several important differences in the fossil. 



The surface answering to o (fig. 1) is usually larger and of a 

 Fig. 3. Fig. 4. 



Fig. 3, under view, fig. 4, upper view, of the distal end of the humerus of a Pterodactyle. 

 Wealden. (Natural size.) 



