390 PROFESSOR OWEN ON THE GENUS DINORNIS. 
relatively, and is maintained through a greater extent of the windpipe, as it seems; 
this dimension is 2 lines, with slight change at parts of the circumference. 
Of this species there is one specimen of a sequence of four rings in the same portion 
of matrix (fig 12), another piece with three rings, and three or four with two rings. 
The extreme of depth of hoop is reached at part of the circumference of the ring 
(fig. 11, a, d). 
§ 10. Trachea of Dinornis elephantopus ? 
The tracheal rings of the third series are remarkable for their great breadth and 
thickness. There are about 80 of these, of a full elliptical, subcircular, or circular 
shape, with an average diameter or long diameter of 9 lines. The specimen figured 
(Pl. XLVII. fig. 13, a, 6) shows the average size or common character of these strong, 
broad, well-ossified tracheal rings. The exterior surface is rugose, the inner one 
smoother, both surfaces straight or even from one margin to the other; the margins 
are flat, as if made by a clean cut, and show irregular perforations, probably vascular, 
of the osseous tissue. The thickness of the hoop is rarely uniform, the difference 
being, in several rings, as great as in that figured in 15, a; there is also, occasionally, 
a variety in the breadth at different parts of the circumference of the hoop, though 
rarely to the extent shown in fig. 16, which, from its small size, may possibly be a 
bronchial hoop. 
§ 11. Trachea of Dinornis ingens? 
About 70 tracheal rings show an average of size and shape as in that of fig. 17,a,,¢; 
the extremes in regard to depth of hoop, in this series, are given in figs. 19 & 20. The 
bone, in all, is of unequal thickness, longitudinally rugose, but unequally so, on the 
outer surface, smooth within (fig. 18, longitudinal section). On the rougher part of the 
ring the bony substance stands out in the form of granules or ridges, the latter running 
in the direction from one margin to the other (figs. 19 & 20, 0,5). These margins 
(figs. 17 & 19, @) are flat or “truncate,” as in the smaller rings (figs. 13-15, a) of the 
present robust type; but here the margin is more uneven, with risings and depressions, 
somewhat irregular, but on the whole at right angles to the outer and inner surfaces. 
In this series were specimens of two partially confluent rings, or of a broad hoop 
twisted upon itself spirally, so as to simulate two hoops. Of these specimens one is 
represented at fig. 21, a, 6, a second at fig. 22; fig. 23 shows more plainly a partial con- 
fluence of the two bony rings. Seven rings of the average size of those provisionally 
attributed to Dinornis ingens (Pl. XLVI. fig. 6) occupy an extent of the trachea 
equalling that which includes thirty-nine in Caswarius galeatus (ib. fig. 5). 
§ 12. Trachea of Dinornis robustus ? 
I have finally to notice the largest specimens in the present collection, which ex- 
emplify the most extraordinary degrees of thickness and strength of bone which have 
been hitherto observed in the windpipes of Birds. 
