CRETACEOUS PTERODACTYLES. 257 



the chalk ; and it closely resembles the character of the " foramen pneumaticum" in a 

 bird's bone, but I am not aware of any in that class which is situated on the back part 

 of the distal end of the tibia. On the opposite side of the bone it presents a concavity, 

 which, however, is deepened by the yielding of the thin parietes of the bone at that part. 



In the crushed specimen, fig. 4, the convex contour of the condyles bounding the 

 deep trochlea, describes three fourths of a circle, and hitherto not any of the few well- 

 preserved articular ends of the bones of the Ptcrodactyles have exhibited this structure. 

 This remarkable trochlear joint may terminate either the femur, or the short and 

 thick metacarpal bone of the wing-finger, from the degree of obliquity of the joint I 

 incline to regard it as part of the latter. 



Figures 6 and 7, PI. 5, exhibit two portions of a long bone of a gigantic Pterodactyle 

 from the Green-sand near Cambridge, the shaft of which repeats the same inequilateral 

 triedral form as that of figs. 1 and 4, in PI. 4. The smaller fragment of Pterodactylian 

 bone, also from the Green-sand of Cambridge, fig. 8, PL 5, indicates, by the strong 

 and broad ridge, that it formed part of the proximal end of a humerus ; either of a 

 younger individual, or of a species not larger than that called Pterodadylus giganteus, 

 by Mr. Bowerbank, and of which some of the long bones are figured in PI. 6. 



The natural length of the difl^erent segments of the wing of the great Ptcrodactyles 

 of the Chalk may be estimated, according to their proportions in better preserved 

 specimens of the genus, if we can gain approximatively that of any one of the bones, 

 and more especially of the humerus. Tliis I have endeavoured to do, with the 

 following results. 



In the Fterodadi/lus macronyx, Pt. crassirostris, Pt. longirostris, the breadth of the 

 distal end of the humerus equals rather more than one fifth of its length, and according 

 to this proportion, the humerus, assigned to Pt. com2iressirostns, PI. 2, fig. 1, may be 

 restored, and would give a total length of ten inches and a half. 



In the Pt. macronyx, the length of the humerus is equal to three fourths of that of 

 the ulna ; in Pt. crassirostris it nearly equals one half ; in the Pt. longirostris it equals 

 two thirds of the ulna ; in Pt. longicaudatus it equals three fifths of the ulna. Taking 

 the mean of these proportions, which is nearly that in the Pt. longirostris, we may 

 assign fifteen inches as the probable length of the antibrachial bones of the Pt. 

 compressirosfris. If the bone, PI. 4, fig. 1, be the ulna of the Pt. Cuvieri, it must have 

 been longer by some inches. 



The species of smaller Ptcrodactyles above cited show a greater difference in the 

 proportions of the metacarpal bone of the wing-finger. In the Pt. macronyx this bone 

 is one half the length of the humerus : in the Pt. longirostris it is at least of equal 

 length with the humerus ; the Pt. crassirostris and Pt. longicaudatus come nearer the 

 Pt. macronyx in the proportions of this bone : we may therefore assign, without 

 hazarding an exaggeration, the length of six inches to both carpus and metacarpus of 

 the Pt. compressirostris. 



