332 
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 cleyation 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 conyexity. 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 (4), the activity there of the ossifying process being further 
exemplified by an unusual degree of granulate outgrowths simulating an exostosis: the 
more conyex part of each hoop (a), where the line of separation remains open, is com- 
paratively smooth. The two rings (Pl. XCIII. fig. 28,@0) have completely coalesced— 
the original separation, showing them to have been of the long variety, being feebly, 
though sufficiently, indicated. These also show a markedly flatter side of the ring 
where the bone is thickest and most irregular. It is to this increase of osseous sub- 
stance that the flattening is due, the smooth inner surface of the same part following 
the course of the elliptical section of the air-passage. Lengthwise these anchylosed 
hoops show a greater longitudinal convexity of the smoother side, and a more feebly 
longitudinal concavity of the opposite side; but this indication of a bend of the wind- 
pipe is better marked in the next anchylosed pair of hoops (fig. 29), although they are 
shorter, showing the common size. In these, at the conyex part of the bend, the 
coalescence is incomplete. 
Figure 30 shows two coalesced rings, where the hoops thin off behind and the bony 
texture is exposed by abrasion. This texture is coarse, and, with the character of the 
truncate margins and of the rough parts of the outer surface, gives the hoops or 
cylinders a cork-like appearance. 
Sections of these tracheal rings (fig. 31, D. ingens}, and fig. 32, D. robustus ?) show 
the varying thickness of the bone at opposite parts of the cylinder, the smoothness of 

