148 MR. W. K. PARKER ON THE STRUCTURE 



Skull of Gavia ridibunda. Mh stage. 

 A little older than the last, the young of this stage yielded me the perfect longitu- 

 dinally vertical section (Plate XXVI. fig. 12) now to be described : it runs close after 

 and further illustrates the third stage. 



The first sight of this object, and indeed of its figure, suggests feathery lightness ; and 

 the whole sum of its specialization, as compared with what is seen in the lower classes of 

 Vertebrata, may be taken as typical of this group (the Carinatae), in which the Meptile 

 so marvellously culminates. 



The actual brain-cavity only occupies one third of the entire length of the head ; the 

 nasal region proper, where the olfactory nerves are distributed, is exactly in the middle ; 

 the ear-capsules run into the hinder arch ; and the eyes are fairly halfway between the 

 other sense-capsules. 



The great mesethmoid (pe) has now reached the interorbital fenestra (iof) above, 

 and behind it the cartilage ends in a retral spike, and in a groove below this lies the 

 olfactory crus. 



Then in front of that bony plate we see how the cranio-facial axis has been gnawed 

 away, and a main and a lesser notch are seen leaving still a thick cartilaginous isthmus 

 connecting the small septum nasi (sn) with the main ethmoid. 



In front of the former plate there is a large open gap before we reach the short, solid, 

 hooked end of the premaxillary (poc). 



The feeble maxillary (mx) is left in situ, and its maxillo-palatine plate (mxp) is seen 

 rising obliquely and leaf-like, on each side of the cranio-facial notch ; inside of it is the 

 forked and notched double vomer (v) ; the rest of the face has been removed. 



Behind, we see what has been already described in the second stage (compare figs. 11 

 & 12), but with this difference, namely, that the bony territories are not hedged in with 

 cartilaginous balks, but have their margins edge to edge. 



The prootic is still distinct ; but the epiotic fragments are now lost in the coadapted 

 faces of the prootic and superoccipital ; also the opisthotic has coalesced with the pro- 

 otic and exoccipital. 



The base of the cranial cavity is an ascending floor, concave below and behind, and 

 convex as it begins to close in in front : the brain sits on the rounded upper edge of the 

 presphenoid in front (Plate XXVI. figs. 16 & 17, ps). 



Here the floor is the original membrano-cranium, the cartilage being stunted in this 

 part ; and any orbito-sphenoidal bones are merely such osseous centres as appear in the 

 fontanelle. In the Powl there are two on each side, supplemented by the orbital plate of 

 the frontal. I see no evidence of any distinct centre here in this species ; in the old 

 bird the bony matter merely runs a little way into the fontanelle from the presphenoid. 



Skull of Gavia ridibunda. 5th stage. 

 In these ripe chicks the sutures are becoming obliterated fast, especially those of the 

 ossified chondrocranium ; but there are still many clear divisional lines in the investing 

 or outer part of the skull. 



