PHYLOGENY OF THE PALZOGNATHA AND NEOGNATH#, 225 
in another it was perforated by an oval foramen, which Parker suggests indicates a 
paired origin. 
No traces of a keel were observed, though such are undoubtedly present in some 
adults (fig. 4 D). 
Ossification of the sternum, according to Gadow, begins during the last week of 
embryonic life. 
In Struthio and Apteryx a pair of pleurostea only are represented ; in Rhea, Casuarius, 
and Dromeus there are a pair of additional centres representing pro-ostea. 
Although there is no positive trace of a keel in the developing “ Ratite ” sternum, it 
is generally assumed that such a structure was present in more remote ancestors, and 
that its loss is the result of degeneration accompanied by decaying, and terminating in 
absolute loss of the power of flight. It is possible, however, that the earliest birds did 
not possess a carinate sternum. The Ratite sterna of the Palewognathw may well 
represent this ancient type. The keel was never largely developed in the sterna of the 
Pterodactyle, and is not always present among the Chiroptera. 
b. Pectoral Arch. 
In Struthio and Apteryx only is there a well-developed pre-coracoid. 
Apteryx, however, presents, as Parker [72 | points out, “ the remarkable circumstance 
that in one species of Apteryx (A. oweni) the coracoid is solid, presenting no coracoid 
fenestra and therefore no procoracoid, while in another species (A. australis) the 
procoracoid is present at a comparatively early age, and is frequently retained in the 
form of a ligament in the adult. In other words, the differences between the shoulder- 
girdle in two species of Apteryx are of precisely the same nature as those distinguishing 
the Struthiones from the other Ratite.” 
In Dromeus and Caswarius and in Rhea the pre-coracoid is vestigial. It may even 
be, as Lindsay has suggested, a new growth of the coracoid rather than the homlogue 
of the pre-coracoid of Struthio. 
Clavicles occur only in the adult Dromeus and embryo Casuarius. 
Petyic ARCH. 
The pelvic arch of the Palwognathe differs from that of the Neognathw chiefly in 
two points :—(1) The great height of the neural spines of the synsacrum (PI. XLIV. 
fig. 5); and (2) the persistent ilio-ischiadic fissure. But though there can be no 
difficulty whatever in distinguishing the Struthious from the Neognathine pelvis, yet 
these cannot apparently be defined in mutually exclusive terms. There is no com- 
prehensive definition that will cut off the one from the other. To keep to the pelvic 
arch of the Struthious birds: it is difficult to find a definition for this which 
shall not be subject to any exceptions; for although the persistent ischiadic fissure is 
