xv, i Shufeldt: The Monkey -eating Eagle 43 



The skeleton of the tail of this eagle is composed of eight 

 free vertebrae, plus a large, quadrilateral pygostyle. The first 

 three vertebras are pneumatic, while the rest of this caudal 

 series does not enjoy this condition. Passing to the harpy eagle, 

 we find that the tail skeleton contains but seven free vertebrae 

 and a pygostyle; the latter is similar to that of the monkey- 

 eating species, but here the leading four vertebras are pneu- 

 matic — the rest are nonpneumatic (Plate III, fig. 1, and Plate V) . 

 These vertebrae, including the pygostyle, are massive in struc- 

 ture, and present the usual ornithic characters of this part of 

 the vertebral skeleton in the aquiline types. 



Pithecophaga possesses a very large quadrilateral pygostyle, 

 the lower portion of which seems to be a coossified terminal 

 caudal vertebra, though now an inherent part of the bone and 

 included in giving to it its peculiar shape. Superiorly and an- 

 teriorly the pygostyle possesses sharp edges, while the posterior 

 one is thickened and rounded. Above its articulation in front, 

 there is a small opening for the terminal of the spinal cord to 

 enter, and below this, a double-faced articulation for the last 

 free caudal vertebra. Just posterior to this the pygostyle is 

 perforated from side to side by a smooth, subcircular foramen 

 of some size (Plate III, fig. 2). The lower half, posteriorly, has 

 the form of an isosceles triangle, with the apex above. Its sides 

 are rounded, and its base is much thickened. Interiorly it is 

 excavated, and the above-described foramen passes through the 

 excavation, while the floor of the excavation exhibits a larger 

 foraminal opening of an elliptical outline, with the major axis 

 in the anteroposterior line. 



In the harpy eagle a stumpy apophysis projects from the 

 superoposterior angle of the pygostyle, while the foraminal open- 

 ings are generally smaller. Throughout the falconine types of 

 the world the pygostyle, while it may vary somewhat in form, 

 possesses the same general characters, is always conspicuously 

 large, and is commonly of a quadrilateral form. 



Bones of the shoulder girdle, or pectoral arch. — Taking into 

 consideration the marvelous power of flight possessed by this 

 big eagle, we need experience no surprise at finding the bones 

 composing this distinctive arch as in all birds of great strength, 

 massive in structure (Plate IV). This is preeminently true, 

 and probably no existing bird of its size possesses bigger and 

 stronger bones in its shoulder girdle than an average eagle; to 

 this statement our aquiline giant of the Philippines forms no ex- 

 ception. As is the case with all the bones of the trunk skeleton, 



