OSTEOLOGY OF BIRDS 



49 



the species rises much above the summit of the articular surface 

 of its companion. Below the fibular ridge, this bone dwindles to 

 its usual styliform dimensions, being compressed from before, back- 

 ward, and running well down the tibial shaft into its lower third 

 to terminate in a free, pointed end, in all of the Cathartidae, except 

 Cathartes a. sep ten trio nalis. The union is very 

 intimate at the lower extremity in the skeleton of Sarcorhamphus. 

 The tibiotarsus has a large cuboid head, but the undulating articular 

 surface at its summit is not profoundly impressed by condylar de- 

 pressions, for the trochleae of the femur, and, indeed, the pro- and 

 ectocncmial ridges are but feebly developed; the latter is produced 

 Mbulaward as a strong though blunt tuberosity, shielding the superior 

 tibiofibular articulation in front. The cncmial crest above these pro- 

 cesses is likewise low and not raised to any extent above the general 

 articular surface to which it forms the anterior boundary. 



A section of the tibial shaft,, made anywhere between the distal 

 extremity and the fibular ridge, shows it to be broadly elliptical, 

 and the entire shaft is bent so as to be convex anteriorly, concave 

 throughout its length posteriorly ; it expands transversely as it ap- 

 proaches the distal extremity, where we find the usual points for 

 examination found in the vast majority of the class. A broad and 

 strong osseous bridge is thrown obliquely across the groove that is 

 the continuation upward of the intercondyloid notch, to retain the 

 extensor tendons. The trochleae are uniform in outline, placed in 

 anteroposterior and nearly parallel planes, the fibular one being 

 the broadest anteriorly. The notch separating them is deepest just 

 below the bony bridge for the extensors in front, while behind it is 

 not carried very far up the shaft and becomes very shallow, the 

 trochleae apparently running into one common surface. 



The 'bones of the leg of Neophron percnopterus are 

 very similar to those found in the Cathartidae ; the principal differ- 

 ences seem to be that the pro- and ectocnemial ridges at the prox- 

 imal and the trochleae at the distal extremity are placed rather 

 farther apart ; the bony span to hold the extensor tendons is the 

 same. We mention this fact because in some of our American 

 hawks and caracaras (Tinnunculus, Polyborus) it is found to be 

 double, i. e. the bridge above is thrown across a wider tendinal 

 groove in these birds, and from the lower margin of the span 

 another bony piece is joined that is carried down to the intercondy- 

 loid notch. This arrangement gives one opening above and two 

 below, one on either side of the last bony span mentioned. 



