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 vertebra 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 vertebra 
and a pygostyle; the latter is similar to that of the monkey- 
eating species, but here the leading four vertebra are pneu- 
matic — the rest are nonpneumatic (Plate III, fig. 1, and Plate V). 
These vertebra, 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 aidiculation 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, v/e 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, 
