206 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 20, 



bone from that of all other known existing animals : save as regards 

 the proximal end, they are sufficiently evident in the tibia of the 

 Gastornis, and permit no manner of doubt as to the class of animals 

 to which that bone belongs. There is, however, a great range of 

 variety in some of these, and of other less constant, characters of the 

 birds' tibia. 



In comparing the distal end of the tibia, attention must be paid to 

 the following points : — Relative breadth and depth of that end : 

 Relative size and shape of the anterior productions of the condyles, 

 fig. 2, a, b, and of the interspace between them : Configuration of the 

 rest of the trochlear surface, fig. 2, a, d : Presence or absence of a 

 bridge, e, completing a canal, /, for the exterior tendon of the toes 

 on the fore-part of the distal end of the tibia : Direction, position, 

 and size of the bridge : Position and aspect of its lower outlet, g ; 

 Entocondyloid, A, and ectocondyloid, % surfaces, — or those that are 

 found on the inner side of the inner condyle, a, and the outer side 

 of the outer condyle, h. 



The distal end of the tibia varies in its degree of expansion as 

 compared with the shaft, in the relative prominence and thickness of 

 the cond3'^les, in the width and depth of the intercondyloid space, and 

 especially in modifications of the anterior surface above that space. 

 This surface is traversed by the * extensor communis ' tendon of the 

 toes, and here the tendon, in all young birds, is strongly bound down 

 by a more or less oblique or transverse ligament, which in most 

 species becomes ossified before full growth is attained : it then forms 

 the * bridge ' or * supra-tendinal bridge,' e. The existing Struthio- 

 nidce, viz. the Ostrich, Tlhea, Emeu, and Cassowary, are exceptions, 

 also the Hornbills and Parrots : in them the ligament retains its nature 

 throughout life. 



Order Raptores seu Accipitres. 



In the Sea Eagle (Haliaetus alhicilld) the breadth of the condyles 

 exceeds the depth* ; the anterior convexities each equal the inter- 

 condyloid depression : the posterior trochlear space is broad and 

 oblique. The bridge is broad, median, and very oblique from above 

 downwards and outwards. Below the bridge a thin transverse rising 

 bounds the intercondyloid depression above. There is a shallow de- 

 pression at the sides of the distal end, above each condyle ; an 

 obtuse tubercle projects from the middle of the inner concavity. 

 The inner side of the shaft is thinner than the outer one, which is 

 contrary to the proportions of the part in most birds. 



In the Vulture {Sarcoramphus papa, fig. 3) the depth and breadth 

 of the condyles are nearly equal ; the inner one is smaller than the 

 outer ; the bridge lies to the inner side of the mid-line, and is more 

 nearly transverse than in the Eagle. A small fossa is defined by a 

 transverse ridge at the intercondyloid space : the tuberosity in the 

 depression above the inner condyle is less developed. The posterior 



* By depth I mean the fore-and-aft, or antero-posterior diameter. 



