PROFESSOR OWEN ON THE GENUS DINORNIS. 391 
I think it not improbable that an osseous hoop like that represented in fig. 24, a, b, 
might, if received as a solitary fossil, have passed rather for a section of the shaft of a 
pneumatic limb-bone, being as large, for example, as such section of the femur of a 
Cassowary, but thicker in the walls. He must have been a bold, as well as acute, 
paleontologist who would have pronounced it one of the rings of a bird’s windpipe. 
I have now, however, received upwards of thirty specimens, averaging the dimensions 
of that of fig. 24. They are, most of them, rather more elliptical, less circular, than 
the smaller hoops of a like type (figs. 17-23). The long diameter averages, as in fig. 24, 
1 inch 2 lines, the short diameter 1 inch, outside measure; the area, which is a more 
regular ellipse, gives 103 lines and 9 lines in the two diameters. The breadth, or we 
may now say the length, of the hoop’s wall, ¢. e. from the upper to the lower margin, 
averages 9 lines and 73 lines, not being uniform all round; the difference of thickness 
is greater, viz. from 23 lines to 3 a line (fig. 24, a, and fig. 32). 
The contrast between the outer and the inner surfaces of the tracheal hoops in 
Dinornis becomes greater as these increase in size. In the present series, which may 
belong to Dinornis robustus, the irregular longitudinal striation prevails over the 
external surface of the bone; but there are other characters. 
At one or two parts of the circumference a part of that surface (figs. 24, 26, 28, x ) pro- 
jects beyond the rest, usually from the middle third part between the upper and lower 
borders; these elevations, or the elevation, if it be single or continuous, are limited to 
one side of the hoop, and to that which is most convex or least flattened. The degree 
of elevation is slight, from a fourth to a sixth of a line; the surface is smoother than 
the parts above and below. ‘These elevations I take to indicate the interval between 
the surfaces of insertion or attachment of fibrous substance connecting one ring to the 
next in a more special manner than the general external investment of the hoops, the 
fibrous character of which may be indicated by the general longitudinal striation of the 
external surface. The smoother part of that surface is usually opposite the side showing 
the broad and low elevation. Besides the foregoing accentuations of the outer surface, 
many of the hoops show coarser granulate outgrowths at the rougher part of the bone. 
In almost all of the present series of rings the longitudinal lay of the outer surface, 
from one margin to the other, if it is not straight, tends rather to convexity. The 
longitudinal lay of the smooth inner surface is more uniformly straight; but there is a 
feeble transverse rise, or linear impression, indicative of a tract on the inside corre- 
sponding to the elevation on the outside of the hoop. 
In the present, as in the preceding series, there are differences of length, breadth, 
and thickness of the wall of the hoops; the two extremes of the first dimension are 
shown in the subjects of figures 25 & 26. There are also six instances of confluence of 
two hoops; in no received example is co-ossification of the tracheal rings carried further. 
Fig. 27 shows two of the shorter variety of hoops coalesced at the flatter and rougher 
half of their circumference (0), the activity there of the ossifying process being further 
3H 2 
