PROFESSOR OWEN ON THE GENUS DINORNIS. 317 



The following are the dimensions of this mutilated but instructive bone: — 



In. Lin. 



Breadth across the articulation of the last rib 9 9 



Breadth between ends of posterior angular processes 13 



Length, or antero-posterior diameter at middle of posterior emarginations . 6 



Thickness at articular margin for ribs 10 



Length of posterior angular process from middle of emargination ... 4 



Extent of costal articular surface 2 C 



Extent of coracoid articular surface 10 



This, the largest sternum which has hitherto been discovered in the class of Birds, 

 is relatively the smallest in proportion to the body, if it belong to the Dinornis giganteus. 

 That it belongs to the genus there is no room for doubt, since it was found associated 

 exclusively with the abundant remains of different species of Dinornis, and especially 

 with those of Din. giganteus. In its small relative size, its shortness as compared with 

 its breadth, its shield-shape and the total absence of a keel, it demonstrates the want of 

 a power of flight in the genus Dinornis, and its closer relationship to the Cursorial or 

 Struthious order. The following are comparisons of the present sternum with the mo- 

 difications of form which that bone presents in the different genera of that strictly ter- 

 restrial order. 



The sternum of the Ostrich {Struthio, PI. XLIIL fig. 4) is larger in proportion to its 

 breadth ; the hind-part is narrower, instead of being, as in Dinornis, broader than the 

 fore-part : the sternum of the Ostrich is more convex, and is a much thicker bone, 

 especially at the middle prominent part of the body of the bone which transmits the 

 weight of the trunk upon the sternal cushion or callosity, upon which the Ostrich rests 

 when prone on the ground. The coracoid grooves {ib. c, c) are considerably larger in 

 the Ostrich, extending from the outer angle, close to the middle of the anterior border, 

 which is thin and sharp : the costal articulations {ib. r) are broader, much deeper, and 

 occupy a much greater proportion of the lateral borders of the sternum. The posterior 

 angles (ib. p) are prolonged backwards, but not so far as in the Dinornis, and there are 

 two emarginations {ib. e, e) on each side between the angles and the middle line, to which 

 a cartilaginous, and, in old Ostriches, a semi-osseous xiphoid appendage {x) is attached. 

 In the Rhea the sternum (PI. XLIII. fig. 5) deviates more than in the Ostrich by its 

 greater length, median convexity and posterior contraction, from that of the Dinornis : it 

 further differs in the absence of posterior angular prolongations and the presence of a pos- 

 terior median marginal notch (e) ; but the coracoid cavities (c, c), though considerably 

 larger than in the Dinornis, are more confined to the anterior angles than in the Ostrich. 

 The coracoid cavities have a similar position, but rather smaller relative size, in the 

 sternum of the Cassowary {ib. fig. 6), which however differs as much as that of the Rhea 

 from the sternum of the Dinornis in its greater length as compared with its breadth, and 

 especially in its contraction to the posterior margin, where the angles are rounded off 

 and the middle part slightly produced. 



