OSTEOLOGY OF BIRDS 



333 



pubic bones in Olor are very much expanded, though otherwise 

 these also agree with the corresponding elements in Branta. 



If the pelves of adult specimens of Olor columbianus and 

 Branta canadensis be placed in contact, back to back, in 

 such a manner that the centers of the acetabula, upon either side, 

 are all in the same plane perpendicular to the axis of the spinal 

 column; and that the longitudinal axes of the .bones also each lie 

 in another imaginary plane at right angles to the first one, then 

 it will, be observed that the pelvis of the swan exceeds in length, 

 anteriorly, the pelvis of the goose by about a centimeter, it being 

 rather more than this behind. Further, it will be observed that 

 the greatest widths, posteriorly, are equal, and within a millimeter 

 or two of it, anteriorly; the first measurement being made op- 

 posite the leading caudal vertebra, the latter opposite the prezyga- 

 pophyses of the first pelvic vertebra. An imaginary line joining 

 the upper points of the antitrochanters measures in the swan 6 

 centimeters, and a corresponding one in the goose measures 5.50 

 centimeters. 



In other words, taking the size of the two birds into consideration, 

 it will be observed that the swan has the longer pelvis, actually, 

 as it is in reality, relatively narrower. 



The general form of the sternum of Olor columbianus 

 agrees with that of Branta canadensis, but to this is to be 

 added, however, some very special characters of its own. Chief 

 among these is the presence in the sternum of the swan of a very 

 capacious intracarinal chamber, that is continued on posteriorly 

 into a pear-shaped, vertically subcompressed, osseous box occupy- 

 ing the dorsal surface of the hinder half of the sternal body. The 

 contracted part of this lies in front ; the swelled part, behind ; where 

 it slightly overlaps the xiphoid notches. This extraordinary cavity 

 is intended to harbor, during the life of the individual, a loop of 

 the trachea, the entire anterior border of the sternal keel being 

 given over for an entrance to it. Upon measuring this aperture, 

 which is elliptical in outline, we find its greatest transverse width 

 to be rather more than a centimeter, while vertically, from manu- 

 brium to carinal angle, it is 4 centimeters. There are nine haema- 

 pophysial facets upon either costal border, and the xiphoid ex- 

 tremity of bone, is once notched upon either side. These " notches " 

 are not more than one fourth the size they are in Branta 

 canadensis, while the mid xiphoid prolongation is not 

 strongly ossified, exhibiting in its surface small vacuities, here and 

 there; its free margin behind being more or less ragged. This 



