148 ME. W. K. PAEKER ON THE STEUCTUEE AND 



backwards, forwards, and a little upwards, the upper lamina of bone in the fundus of 

 the "sella" is growing upwards right and left into the clinoid bridge and posterior 

 pterygoid processes, and backwards along the floor of the skull. The two wings of 

 this upper bony lamina only slowly meet behind the " sella," both above in the great 

 transverse bridge, and below in the postpituitary skull-floor ; hence the slit which for a 

 long while is seen at the mid line in this and other birds. In the side view, the slit 

 which separates the high prepituitary mass from the root of the rostrum below (Plate 

 XIII. fig. 2, b.s., r.b.s.), shows how that this rostrum is an extension of the secondary 

 growth of cartilage below the " infundibulum," and altogether distinct from the trabe- 

 culse ; the late-formed cartilage of this floor "and its rostral prolongation, however, is soon 

 formed into a continuous mass with the cartilaginous trabeculse. A front view (Plate 

 XIII. fig. 7), supposing the rostrum to be severed at its free part, shows the structure 

 of this bone well, and also that the posterior pterygoid processes lie on a higher plane 

 than the spurs in front of them. This extension outwards of the great basisphenoid 

 is a correlate of the transverse position of the alisphenoids, which are best seen, not 

 at the sides, as in most vertebrata, but in front (Plate XIII. fig. 7, a.s.), as they form 

 the large posterior walls of the orbit. These irregularly pentagonal bones are only 

 gently convex at or near the outward (posterior) margin ; much of their orbital surface is 

 smoothly flat as they approach, thin and splintery towards the small anterior sphenoid. 

 Although subpentagonal, each side of the alisphenoid is rounded or convex ; the external 

 and lower margins are thick ; the outer (upper) angle has a feebly expressed epiphysis, 

 the postfrontal {p-f-); whilst the lower margin is deeply scooped near the inner and 

 the outer cornu ; with the help of the basal bones, these notches are converted into the 

 foramina ovalia (5), and rotunda (5, a). Between these bones and below and behind the 

 presphenoid is the large semicircular "common optic foramen" (2); the anterior sphe- 

 noid is connected with the alisphenoid by mere fibre (Plate XIII. fig. 7, p.s.), a remnant 

 of the original membranous skull, and now forming part of the "postorbital fontanelle" 

 of each side ; a remnant of this exists in the adult ; periosteal layers of bone have filled 

 up the rest. The presphenoid and ethmoid (Plate XIII. fig. 5, eth.) are still separate ; 

 the two moieties of the latter have quite coalesced ; it is a large bone ; its alse are ossi- 

 fied almost as far as in the adult ; and the fenestra is somewhat more circumscribed by 

 periosteal layers. The bone has still territory, fore and aft, in a cartilaginous condition, 

 but it has reached the rostrum below ; seen from behind (Plate XIII. fig. 6) the olfac- 

 tory notches show themselves above that upper bar, which ultimately joins the presphe- 

 noid. The lower part of the turbinals (antorbital, "pars plana vel papyracea") is still 

 wholly soft, and wholly separate, except by a fibrous connexion from the perpendicular 

 plate. 



The fenestra in the antorbital plate of the lacrymal is well shown (Plate XIII. fig. 8); 

 as also the splints of the lower jaw (Plate XIII. figs. 9-11). Whilst much of the outer 

 and anterior part of the articular cartilage is still unossified, yet at the apex of the 

 internal angular process (Plate XIII. fig. 9, p.fo.) the pneumatic foramen is well formed, 



