PIITLOGENY OF THE PAL.EOGXATH.E AND NEOGNATILE. 199 



Casuarhis differs conspicuously from Lroinwus and the otlier forms associated here- 

 with in that, that portion of the mesethmoid which in Lromceus, for instance, reaches 

 tlie 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 

 oi the nasals and that portion of tlie frontal bone which comes into juxtaposition with 

 the nasal and median masses (PI. XLIV. figs, o & 4 a-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 tliis 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 tlie 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 PI. XLIV. figs, o 

 and 4 a-b. 



In Rhea and the Tinamous, as in Dromceus and Casnarias, the dorsal border cjf the 

 mesethmoid comes into chjsc 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 tcgminal process. But the passage for the olfactory nerves is 

 restricted to an exceedingly narrow space lying on either side of this process. In 

 Dromieiis and Casnarias tliis passage, though formed in precisely the same way, is 

 much larger. 



In Ajjteri/x 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 Casnarias or even iJromaas. The tegminal process is less well-developed. In 

 the embryo, it appears on the surface of the skull as in Uronueas, 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 quadrate in the embryo of Dronueas differs in shape from that of the adult in 

 the lack of an ossified orbital process, and in that the external mandibular condyle 

 is <-siiapcd instead of transversely oblique. 



In Apteryx that of the embryo appears to differ in nowise from that of the 

 adult. 



The other skulls in the jNluseum collection are too advanced to afford any intermediate 

 characters. 



The articalar 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 



