PROFESSOE OWEN ON THE GENUS DINORNIS. 121 



prominences (PL XIV. fig. 2, a, a), from wliich 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 bii-ds of flight. The body of the sternum describes a slight curve lengthwise to the 

 tapering end, with the convexity do\\'nward 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 Aiitornis otidiformis 1) 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. pi. 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. PI. 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 o£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 Ajit. otidiformis (1). 



The second, third, and fourth articular surfaces for the sternal ribs are ridges with 

 extensive intervening smooth and imperforate cavities (PI. XIV. fig. 4, c, c). The last 

 or hindermost articulation is a small subcii'cular 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,r) 

 for the insertion of muscle is bounded by a feeble ridge (ib. fig. 2, t, r) 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 bone. 



At the fore part of the inner or upper surface the sternum is strengthened by a 

 transverse prominence or bar (ib. fig. 3, h), which expands as it subsides at each end 

 upon the inner part of the costal tract. This bar is fui-ther from the anterior border, and 

 consequently more internal in position, than is the corresponding ridge in the sternum of 

 Apt. otidiformis (1), 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, t), concave forward, nearly midway 

 ' Trans. Zool. Soc. vol. iv. p. 18. ' lb. vol. iv. pi. 4. fig. 8. 



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