16 Osteology of Circus hudsonius. 
nineteenth vertebra inclusive. This pair of openings are largest 
in the eighteenth vertebra, and smallest in the thirteenth. 
The ‘‘ intervertebral foramina’’ become more circular and yet 
smaller as we proceed toward the hinder part of the spinal 
column. 
In the fifteenth vertebra the neural crest interlocks at its posterior 
superior angle with the anterior superior angle of the neural crest 
of the sixteenth vertebra, by the arrow-head joint. This inter- 
locking continues throughout the series until we arrive at the 
pelvis, where no such joint is found to exist. The neural spines 
or crests through this ‘‘ dorsal region’’ of the column become 
gradually lower and longer as we proceed towards the posterior 
extremity of the body. 
From the fifteenth to the nineteenth vertebra inclusive, the artic- 
ular facets on the zygapophysial processes gradually change their 
direction to meet the requirements of the ‘‘ dorsal region,’’—they 
once more come to face directly upwards anteriorly, while the 
reverse holds good behind ; we observe also that the transverse 
processes in this series become longer and longer as we proceed in 
the same direction, and their outer extremities armed in each case 
with a single, delicate metapophysis which overlaps the process 
both before and behind it. In the fifteenth vertebra, now under 
consideration, the hypapophysis loses its tricornute character, and 
the short pedicle merely supports a circular disc, with its inferior 
surface directed slightly forwards. This pedicle in the sixteenth 
becomes longer, and the disc becomes an ellipse, placed longitu- 
dinally upon it. ‘The hypapophysis on the seventeenth vertebra 
dips well down into the pleural cavity as a laterally compressed 
hook with slightly dilated apex. It is truly claw-shaped in the 
eighteenth vertebra, though still compressed from side to side, to 
be entirely absent in the nineteenth. 
In Circus all the vertebree are freely articulated upon each other 
as they are in the owls, from atlas to the one that first anchyloses 
with the ilia ; in alco sparverius, however, from atlas to thirteenth, 
inclusive, are free, while fourteenth to eighteenth are thoroughly 
fused into one bone, the outer angles of their diapophyses even 
being united by anchylosis. 
In this common ‘‘dorsal’’ piece of the Sparrow Hawk the two 
leading vertebree support hypapophyses. ‘These have also fused 
together, leaving only a circular foramen between them. ‘The 
