PHYLOGENY OF THE PAL.'EOGNATH.E AND NEOGNATH^. 177 



In many Dinormthida' these apertures lie closely packed together down the mouth 

 of a deep lacerate fossa. 



In most of the other Pald'ognntJm more or fewer of these foramina become more 

 or less confluent one with another and with the optic foramen. 



The EtlimoiiJal Region. — The mesethmoid, in all but Dinornithidce. and Apteri/x, 

 forms the thin median septum known as the interorbital septum. It is continued 

 forward into the pre-lachrymal fossa as far as the cartilaginous septum nasi. Its 

 ventral border rests upon the parasphenoidal rostrum ; its dorsal border affords support 

 to the frontal and nasals. This it does by means of paired, lateral, horizontal, ecto- 

 ethmoidal plates. These, the sitpen'or aliethmoids, curve outwards and downwards, 

 as the inferior aliethmoids, to form the walls of the olfactory chamber. It is 

 perforated superiorly in tlie dried skull so as to place the two olfactory chambers in 

 communication in all but Apteryx, Casuarius, and Crypturi. 



In Dromwiis and Casuarius tlie whole of the outer wall of the aliethmoidal region — 

 the only region that ossifies — is deeply invaginated. The lower region of this in- 

 vaginated wall forms a large, outstanding, antorbital plate, to the outer free edge of 

 which the lachrymal is closely applied (PL XLIV. lig. 1 a). The olfactory chamber is 

 thus reduced to a narrow slit-like cavity, whose outer wall is scroll-shaped, the scroll 

 being formed by the invagination before mentioned. 



This invagination lodges the Harderian gland, and is called therefore the Hardeiian 

 fossa. Opening above and behind this is a small supra-orbital fenestra. 



In Rliea the invagination of the aliethmoidal wall is less sliarply marked, and the 

 Harderian fossa, though larger, passes gently backward into the orbit. Except in 

 very old specimens, the greater portion of the wall of this fossa remains unossifled. 

 The supra-orbital fenestra, when ossification is complete, is large. 



In Struthio only the superior aliethmoidal wall is ossified. The postero-inferior 

 region forms the antorbital plate; this is continued upwards and forwards to form 

 the roof of the olfactory chamber. There is a supra-orbital fenestra. 



In Crypturi the antorbital plate stands out at right angles to the mesethmoid as 

 a narrow bar of bone. The upper portion of the aliethmoidal wall resembles that 

 of Rliea in being deeply invaginated, so much so as to force it inwards almost on to 

 the mesethmoid, thus forming a huge Harderian fossa and reducing the olfactory 

 chamber to the smallest possible limits. There is a well-marked supra-orbital fenestra. 



In all the Paheognathce, save the Crypturi, the ossified ectoethmoidal plate extends 

 backwards, on either side of the dorsal border of the interorbital septum, as far as the 

 brain-case. Thus only the lower region of this septum is visible in the skeleton, not 

 its whole extent as in Neognathce. This feature is most noticeable in the Dinornithidce 

 and in Apteryx. In these the olfactory chambers are of enormous size, extending 

 backwards nearly or quite as far as the optic foramina, thus so far encroaching 

 upon the orbit as to obliterate the interorbital septum. This last is traceable only 



