Paeker. — On Notornis mantelli. 249 



and 4a (pi. XXI.), the former showing the outHne of the sternum of 

 Notornis from the ventral face with that of Ocydronius superposed in dotted 

 outline on the left side, and that of Porphyria in broken outline on the 

 right ; while fig. 4a shows the right half of the sternum of Notornis with 

 that of Trihonyx superposed in dotted outline. 



The sternum of Notornis is broad and flat ; its anterior edge is somewhat 

 emarginate, as in Ocydromus, and is devoid of all trace of the manubrium or 

 rostrum (fig. 4, 4a, r) found in Porphyria, and to a less degree in Trihonyx. The 

 coracoid grooves are even more widely separated than in Ocydromus, instead 

 of having merely the width of the rostrum between them, as in the other 

 genera. The diminution in width of the sternum from its anterior to its 

 posterior end is very gradual ; in this respect Notarnis most nearly approaches 

 Trihonyx. The external xiphoid processes {e. x.p.) are divergent, not expanded 

 at their distal ends, and are proportionally shorter than in either of the 

 allied genera ; the middle xiphoid process is blunt and unossified, the bone 

 terminating in a straight transverse edge, about six mm. from the actual 

 extremity of the process. In this again the resemblance between Notarnis 

 and Trihonyx is of the closest kind : the middle xiphoid both in Porphyria 

 and Ocydromus is completely ossified, terminating in the former by a trun- 

 cated edge, while it is deeply emarginate in the latter. 



The keel of the sternum is feebly developed, being hardly deeper, pro- 

 portionally to length of trunk, than that of Ocydromus. Its anterior edge 

 has nothing of the strong forward convexity found in Porphyria, but passes 

 almost insensibly into the ventral edge ; in this respect the resemblance to 

 Ocydromus would be great, but for the fact that in the latter a strong bifid 

 thickening (fig. 4, k) is formed at the junction of the anterior and ventral 

 borders, whereas the corresponding thickening in Notornis is less marked 

 and shows no tendency to division ; the resemblance to Trihonyx is here 

 very marked. 



Another point connected with the Sightlessness of Notornis is the very 

 sHght lateral cm'vature of the sternum; its two sides enclose a dihedral 

 angle (fig. 5 b.) which is nearly as open as that of Ocydromus (a) and con- 

 siderably greater than in Trihonyx or Porphyria {c andn). Tlhis transverse 

 sternal angle as it may be called, seems to be pretty constantly more open in 

 flightless birds than in the normal members of the same group ; its increase, 

 and the correlated diminution of the keel, cause the sternum to approach 

 to the ratite type, as is especially well seen in Didus, Cnemiornis, Stringaps, 

 and Aptornis, and to a less extent in Nesonetta, Ocydromus and Notornis. 



On the whole the sternum of Notornis differs from that of Trihonyx in 

 much the same way as the latter from that of Porphyria. Trihonyx is, in 

 all important respects a mean between the two extremes furnished by 

 Porphyria and Notornis. Ocydromus, on the other hand is, in some respects, 



24 



