PROFESSOR OWEN ON THE GENUS DINORNIS. 121 
prominences (Pl. XIV. fig. 2, a, a), from which the faint beginnings of the keel converge 
as they retrograde to the thick low ridge (ib. s, s) representing that part of the sternum 
in birds of flight. The body of the sternum describes a slight curve lengthwise to the 
tapering end, with the convexity downward or outward (ib. fig. 4); the general trans- 
verse lay of the outer surface of the sternum is slightly convex; but between the 
keel-ridge and the lateral margin the surface is feebly excavated. The ridge (ib. s, s) 
expands and subsides about an inch and a quarter from the hind end. 
In the smaller sternum (of Aptornis otidiformis?) the initial ridges converging from 
the anterior tuberosities to the sternal ridge are better defined by the excavation of the 
surface outside them than in Aptornis defossor, and the ridge expands into a trans- 
versely convex prominent tract, which is continued to the broken end (as shown in 
fig. 5. pl. 4, Zool. Trans. vol. iv.); it consequently extends further back than in Aptornis 
defossor, apparently not leaving a terminal flattened tract of the outer surface of the 
bone, such as is shown in fig. 2. Pl. XIV., near the end of the sternum. The costal 
border in Aptornis defossor forms two fifths of the lateral border of the sternum, and 
presents articular surfaces (ib. fig. 4, c, c) for five sternal ribs. The foremost surface is 
a narrow ridge, crossing obliquely the costal surface close to the articular groove for the 
coracoid. This surface appears to be obliterated by anchylosis of the coracoid to the 
sternum on the right side (ib. fig. 2,d); and the left side shows a fracture at the part 
corresponding to the smooth deep coracoid groove in the sternum of Apt. otidiformis (?). 
The coracoid union with the sternum is restricted to as small an extent, relatively, of 
the anterior border or base of the bone as in Apt. otidiformis (?). 
The second, third, and fourth articular surfaces for the sternal ribs are ridges with 
extensive intervening smooth and imperforate cavities (Pl. XIV. fig. 4,¢,¢). The last 
or hindermost articulation is a small subcircular cavity with a raised border. ‘The 
costal border contracts from the third ridge backward. ‘The non-articular side-border 
of the sternum contracts as it recedes to a rather sharp edge, the two sides converging 
to a nearly pointed end. There is no trace of the lateral fissures and slender processes 
characterizing the sternum of Ocydromus or Tribonyx’. A very shallow tract (ib. fig. 2,7) 
for the insertion of muscle is bounded by a feeble ridge (ib. fig. 2, ¢, 7) 8 lines behind the 
anterior border of the sternum, which tract is less smooth and even than the rest of the 
outer surface of the bane. 
At the fore part of the inner or upper surface the sternum is strengthened by a 
transverse prominence or bar (ib. fig. 3, 0), which expands as it subsides at each end 
upon the inner part of the costal tract. This bar is further from the anterior border, and 
consequently more internal in position, than is the corresponding ridge in the sternum of 
Apt. otidiformis (2), where it seems to form the upper or convex border of a thickened 
anterior margin of the sternum’. A shallow excavation of the inner surface of the 
sternum is bounded by a curved border (ib. fig. 3, ¢), concave forward, nearly midway 
1 Trans. Zool. Soc. vol. iv. p. 18. 2 Tb, vol. iy. pl. 4. fig. 8, 
v2 
