6 ON THE SYSTEMATIC POSITION OF HESPERORNIS. 
The union of the mandibles at the symphysis by ligament is a feature quite 
peculiar to Hesperornis: but the shape of the mandible, the character of its articular 
surface, and the oblique truncation of the posterior extremity are all as in Colymbus. 
In the Ratites the posterior extremity is truncated in the opposite direction, the 
angle being bevelled off instead of prolonged bacixwards. 
To sum up as regards the skull, we have in Hesperornis, as it seems to me, no 
single Ratite character : but, on the other hand, in detail as well as in generai form, 
particularly in the palatine region, the pterygoid, quadrate, squamosal, and nasal 
bones, in the supra-orbital depressions, in the inter-orbital septum, and in the 
articular extremity of the lower jaw, we have characters unknown in the Ratites, 
and in most cases identical with those of Colymbus. 
THE VERTEBRAL COLUMN. 
The vertebrze of birds are so variable in number, that the large number present 
(forty-nine) in Hesperornis, cannot be made a point in classification ; greater than 
that in Colymbus, it is considerably less than in any Ratite except Apteryx. 
The atlas vertebra is missing in the specimens described by MArsu. 
The axis in the Ratitee* has certain definite general characters. Though 
immensely larger than the atlas, it is small (except in Apteryx) compared with the 
third vertebra. The ridge between the anterior and posterior zygapophysis is per- 
forated longitudinally by a canal or foramen; and the ventral or heemal process is 
either absent, or is short, rounded, and blunt. In Colymbus, as well as in 
Hesperornis, the axis is as large and as long as the third vertebra, and there are 
Fig. 5. Axis VERTEBRA OF H&ESPERORNIS. 
1, Lateral. la, inferior. 1b, anterior view. 
* Compare with what is here said about the vertebra of the Ratites the description and figures of 
Mivart (Trans. Zool. Soc. VIIL., pp. 385-451, 1874; and X., pp. 1-92, 1879). 
