102 MR. W. K. PARKER ON THE STRUCTURE 



descending nasal septum becoming solid at its sides continuously with the crest that 

 grows upwards from each trabecula. The broadest part of the "trabecular commissure " 

 (Fowl's Skull, plate lxxxi. figs. 1 & 2, tr) develops into two important structures, 

 namely, the subnasal floor (plate lxxxii. fig. 11, s.v.l) and the conjugational process, 

 which corresponds with the " ethmo-palatine," or orbital plate of the palatine. The sub- 

 nasal or " supervomerine " laminae are of small extent and temporary in the Fowl 

 (plate lxxxii. fig. 11, s.v.l) ; and the conjugational process does not develop into a 

 distant structure, but remains as part of the antorbital wall, its infero-external angle 

 (plate lxxxiii. fig. 5, p.p). In many birds, however, it becomes separately ossified as the 

 " os uncinatum " of Magnus, and takes its place as a kind of transverse bone in the 

 " Musophagidse " and other " Coccygonaorphse "*. 



In the first of my new illustrations of the Chick's Skull (Plate XX. fig. 1, tr, pn), 

 the down-bent skull of a chick, intermediate between the first and second stages of my 

 former paper, has been sliced off in an obliquely transverse manner, cutting away the 

 fore part of the projecting optic lobes (C 2 ) and thalamencephalon (C la ), and removing 

 most of the rudimentary hemispheres (C 16 ). Underpropping the primary vesicle (C la ) 

 are two thick cartilages, which are severed at their commissure ; these are the trabecule 

 at their bend backwards. Prom this position they send downwards an azygous sharp 

 keel. This is the nasal cartilage ; it looks backwards. 



In the next stage (" Fowl's Skull," plate lxxxi. fig. 3, p.n) it is still retral, but has 

 become thick and rounded. 



In another section (fig. 2), taken at a different angle to the axis of the head and seen 

 from its front face and not from behind, as in the last figure, the trabecular knuckle 

 has been cut off, and with it the prenasal cartilage. Here the under-placed nasal sacs 

 {ns) are exposed, and the hooked trabeculse are cut through so as to expose them in 

 section, both in the meso-ethmoidal region above, and under the outer nostrils below. 



In this section the fore part of the eyeballs (e) and the hemispheres (C 16 ) are cut 

 through ; the floor of the nose shows cartilage in it only where the cornua trabecula? are 

 cut through; the prenasal has been pared off. In this rudimentary nasal labyrinth 

 rudiments of the superior and inferior turbinals (utb, itb) are seen, and the membrane 

 infolding them is the early condition of the aliethmoidal and aliseptal cartilages. The 

 eyeball is seen to rest on a very neatly formed ledge ; this is the jugo-maxillary eleva- 

 tion ; and on the side of this is a lesser ridge, which contains the second preoral bar, 

 the palato-pterygoid. 



This palatine bar (Pa) is composed at present of indifferent tissue ; it is triangular in 

 section, and the ascending crest is its ethmo-palatine region ; the conjugational process 

 is correlated to that of the trabecule. 



In a somewhat more advanced skull, sixth day (figs. 3 & 4), the eyeballs have been cut 

 through at their middle and the hemispheres at their hindermost part. This belongs to a 



* See Dr. Reinhardt's paper, " Om en hidtil ukjendt Knogle i Hovedekallen hos Turakoerne." Copenhagen, 1871- 

 In my paper on the " Ostrich's Skull " (p. 158), I referred the " os uncinatum " to the ethmoidal category ; but no 

 explanation of its true meaning was had until I saw the secondary bud growing from the outside of each trabecular 

 bar in the Salmon. 



