Osteology of Circus hudsontus. 25 
bones are compressed from side to side, and diminish in breadth as 
they approach the point of union below. 
Here the clavicles support a small tuberous hypocleidium, which, 
owing to the backward curvature of the fourchette, is about oppo- 
site the coracoidal beds on the sternum. A well developed os 
humero scapulare is supported in the usual manner at the back 
part of the shoulder joint. It is quite a characteristic of the rap- 
torial as well as other groups of birds, and is of great service in 
increasing the osseous articular surface for the humerus. 
Of the Pectoral linb.—Circus in common with many other 
diurnal raptores and Owls has but one pneumatic bone in the 
skeleton of its wing, and this is the humerus (Fig. 11). This 
bone is thoroughly permeated with air, and although of good size, 
it is very light indeed. The pneumatic fossa is of an elliptical 
outline, occupying its usual site, and at its base numerous pneu- 
matic perforations occur. Over it curls the ulnar tuberosity, form- 
ing for it contracted margins on three sides, making the entrance 
smaller than the fossa inside. 
The articular tuberosity for the glenoid cavity is spindle-shaped 
and not very extensive. A decided valley divides it from the ulnar 
tuberosity. Bending over towards the palmar aspect of the bone 
we observe a prominent radial crest. ‘lhis extends from the upper 
end of the articular tuberosity, 3 centimeters down the shaft. In 
form it is a long isosceles triangle, with the angle above, and the 
base on the shaft, (the bone being alongside the body of the bird in 
a position of rest, the one it occupied as I describe it). Viewed 
from above in this position, the humerus has the usual long / form. 
Smooth and cylindrical, the middle third of the shaft presents 
nothing of special interest. Distally, it dilates as usual to sup- 
port on its palmar aspect the radial and ulnar tubercles; a mus- 
cular tuberosity occurs above each of these for tendinal insertion. 
A broad, deep valley is behind the oblique and ulnar tubercle 
occupying the anconal and distal extremity of the bone, to guide 
the passage of tendons to the antibrachium. 
The radius of Circus has a length of 11 centimeters, being a 
slender and nearly straight bone. Its head presents an elliptical, 
concave facet for the oblique tubercle of the humerus, of consid- 
erable size, while the facet for the ulna about its head is not so 
extensive. Just below this latter is the tuberosity for muscular 
insertion. 
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