80 Mk. C. W. ANDREWS ON THE SKULL AND 
the lower end forms the inner border of the very deep groove for the extensor tendons ; 
it terminates at the inner end of the extensor bridge. This latter lies obliquely; near 
the upper border of its inner end there is a deep pit for the attachment of the oblique 
ligamentous sling through which the tendon of the tibialis anticus passes: the other 
end of the ligament is inserted lower down on a well-marked tubercle just above the 
outer condyle; the lower border of the bridge passes externally into a prominent 
tuberosity, such as occurs in the Storks, &c., and serves for the insertion of the ligament 
bridging the extensor tendons. 
The condyles are about equal in size, and are separated by a deep intercondylar gorge 
which opens above into a median concavity lying in the middle line immediately below 
the last-mentioned tuberosity, and, when the leg is flexed, receiving the large inter- 
condylar tuberosity of the metatarsus. Posteriorly the intercondylar groove is short and 
shallow, but is still sharply defined, being bounded by the prominent posterior projections 
of the condyles. Looked at from the side the outer condyle is nearly circular in 
outline, while the inner is much elongated from before backward ; both are concave 
on their outer surfaces. 
The upper end of the bone is relatively small, but the pro- and ectocnemial crests 
are fairly well developed and rise considerably above the proximal articular surface, 
which slopes upward and forward. The inner glenoid surface is flat or only 
slightly convex; posteriorly it forms a considerable projection and is separated by 
a notch from the outer or fibular surface, which is small and very convex in all 
directions. 
There are no pneumatic foramina. The centre of the shaft is occupied by a large 
smooth-walled cavity, and the thickness of the bone in the middle of the shaft does 
not exceed 4 mm. 
The tibio-tarsus of Diomedea differs from that of the fossil in the much greater 
breadth of the intercondylar fossa, the nearly transverse direction of the extensor 
bridge and the absence of a tuberosity at its outer end, and in possessing a much 
larger cnemial crest. 
In Grus the tibia differs widely in the relatively greater width of the distal 
articulation, in which the condyles are small and the intercondylar groove broad; on 
the other hand, there is a median tuberosity. 
In Psophia the extensor bridge is much more transverse, and the outer condyle 
considerably the larger, as in Rails. 
In Cariama the condyles are relatively smaller and the intercondylar groove 
shallower and broader. 
In Ciconia the tibia presents considerable resemblance to the fossil, but the 
articular surfaces for the femur are less distinctly separated, the extensor bridge is 
less oblique, and the groove for the tendons occupies the whole width of the shaft. 
In the Accipitrine birds, including Serpentarius, the cnemial crests are very small, 
