PHYLOGENY OF THE PALZOGNATHA AND NEOGNATHA. 199 
Casuarius differs conspicuously from Dromaus and the other forms associated here- 
with in that, that portion of the mesethmoid which in Dromeus, for instance, reaches 
the outer surface of the skull to form the lozenge-shaped plate already described, is 
surmounted by an independently ossifying mass forming the centre of the characteristic 
casque of the adult—the lateral portions of the casque being furnished by the inflation 
of the nasals and that portion of the frontal bone which comes into juxtaposition with 
the nasal and median masses (Pl. XLIV. figs. 3 & 4a-h). The median portion often 
extends so as to cover the parietal. 
In the adult this casque is seen to be built up of a homogeneous mass of cancellated 
tissue of great delicacy, and protected in the living bird by a horny sheath. The form 
and development of this casque varies considerably, and is one of the factors employed 
in the determination of species. . 
The exact homology of this casque has yet to be made out. That the lateral 
portions are simply inflations of the nasal and frontal bones there can be no doubt, but 
the origin of the median portion is not so certain. It appears to be an independent 
structure superimposed upon the mesethmoid, rather than an outgrowth of that element. 
Stages in the growth and composition of this casque are shown in Pl. XLIV. figs. 3 
and 4 a-b. 
In Rhea and the TVinamous, as in Dromeus and Casuarius, the dorsal border of the 
mesethmoid comes into close relation with the orbital plate of the frontal, but the 
relationship is even closer, this border being narrower, so that the crista galli is 
formed only by the tegminal process. But the passage for the olfactory nerves is 
restricted to an exceedingly narrow space lying on either side of this process. In 
Dromeus and Casuarius this passage, though formed in precisely the same way, is 
much larger. 
In Apteryx the dorsal border of the mesethmoid is exceedingly narrow and _ knife- 
like, and slightly concave. It is, moreover, relatively much shorter antero-posteriorly 
than in Casuarius or even Dromeus. ‘The tegminal process is less well-developed. In 
the embryo, it appears on the surface of the skull as in Dromeus, and also, as in this 
form, sends out horizontal plates which form a lozenge-shaped shield between the frontal 
and nasal bones. Later in life all trace of this shield is obliterated. 
The guadrate in the embryo of Dromeus differs in shape from that of the adult in 
the lack of an ossified orbital process, and in that the external mandibular condyle 
is <-shaped instead of transversely oblique. 
In Apterya that of the embryo appears to differ in nowise from that of the 
adult, 
The other skulls in the Museum collection are too advanced to afford any intermediate 
characters. 
The articular is a pyramidal nodule of bone, bounded cephalad by the coronoid 
the base of which conceals its anterior face; externally (in Rhea) by the supra- 
2F2 
