CRETACEOUS EORMATIONS. 103 



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 Pterodactyles have exhibited this structure. 

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

 thick metacarpal bone of the wing-finger. 



Figures 6 and 7, T. XXXII, 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 T. XXX. The smaller fragment 

 of Pterodactylian bone, also from the Green-sand of Cambridge, fig. 8, T. XXXII, 

 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 

 Pteroclacti/Ius giganteus, by Mr. Bowerbank, and of which some of the long bones are 

 figured in T. XXXI. 



The natural length of the different segments of the wing of the great Pterodactyles 

 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. This I have endeavoured to do, with the 

 following results. 



In the Fterodadylm macroni/w, Pt. crassirostris. Ft. longirostiis, 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. compressirostris, Tab. XXIV, fig. I, 

 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. lonrjirostris 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. 

 compressirostris. If the bone, T. XXX, fig. 1, be the ulna of the Pt. Cuvieri, it must 

 have been longer by some inches. 



The species of smaller Pterodactyles above cited show a greater diflFerence 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. lonyirostris 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. 



