Osteology of Circus hudsontus. 31 
trochlee, there is a rounded and pronounced crest, much like the 
tibial crest in some vertebrates. 
The outer aspect of this bone is broad and flat, being the broad- 
est at mid-shaft. The inner, and at the same time in part the 
anterior aspect is likewise broad and flat, though carried to a sharp 
edge above on the inner aspect proper. ‘This latter side is not as 
broad as the outer aspect. These two surfaces meet to form the 
anterior crest described above. From between the hypotarsial 
processes to the trochleze, the posterior surface of the shaft is deeply 
and longitudinally grooved. 
The margins of this excavation are sharp throughout their ex- 
tent, being the posteior edges of the two surfaces described as the 
inner and anterior and the outer surfaces, above. 
Upon the lower edge of the inner of these two margins we find 
a well marked elongated facet, intended for the articulation of a 
large-sized metatarsal. ‘This latter bone is of considerable size in 
this Hawk, and its distal trochlear surface is very broad, being 
placed transversely on the bone. Above, it is so articulated as to 
allow of considerable freedom of movement, being attached to the 
tarso-metatarsus in the most usual.manner by ligament. This 
distal end of the tarso-metatarsus, that bears the trochlear facets, 
is much expanded in a lateral direction, being gently convex from 
side to side, anteriorly where it shows the usual foramen for the 
anterior tibial artery. Behind, it shows an amount of concavity, 
from side to side, equal to the convexity of the anterior aspect. 
‘The trochlear processes for the pedal digits are separated by not 
very deep notches. ‘Their lower surfaces are about in the same 
plane, the inner one perhaps being rather lower than either of the 
others, though not noticeably so at first sight. 
The mid-trochleze presents a deep median antero-posterior 
groove, not well marked in either of the others. In comparing 
the hypotarsial processes of the tarso-metatarsus as they occur in 
Circus, as I have endeavored to describe them, above, with the 
same processes as they are found upon the tarso-metatarsal of 4 szo 
wilsontanus and alco sparvertus, some interesting points become 
apparent. ‘The arrangement in so is much the same as we find 
it in C7’rcus, there being a single foraminal perforation, while the 
pedicle of the inner process in .4szo is comparatively a little deeper 
from above, downwards. In the Sparrow Hawk, however, we are 
met by a very different state of things. Here we find the outer 
