or Toothed Birds of North America. 261 
long, but was wholly without a keel. The pelvis was greatly 
elongated and somewhat resembles that of the Grebes. The 
acetabulum, instead of being a mere ring of bone, as in most 
birds, is wholly closed internally, with the exception of a fora- 
nfen perforating the inner wall. The posterior extremities of 
the illum, ischium and pubis are free, as in some Ratite and in 
Tinamus. The tail is long, being composed of twelve verte- 
ree, a greater number than is found in any recent bird, except 
possibly thé great Auk (Alca impennis). The middle and dis- 
as 
restricted, so that the principal motion of the tail must have 
been vertical, and it was no doubt a powerful aid in diving. 
The so-called plowshare bone of modern birds is represented 
Y the codssification of the last three or four vertebrae, w ich, 
resistance presented to the water. The remaining digits fol- 
lowed behind the fourth, and the foot was not expanded until 
_ pi&sent species. Hesperornis, as we have seen, was an " 
tle diver, while ai i neck with its capabilities of rapid 
