AND DEVELOPMENT OF THE BIRD'S SKULL. 145 



Behind the inferior turhinal, and also above it, is the coil of the upper turhinal (utb), 

 which is merely the aliethmoidal region of the nasal roof turned in upon itself. 



This is represented as cut through the middle in the inverted figure (fig. 7, utb) ; this 

 and the front wall of the pars plana (see fig. 15), which has no definite middle turbinal on 

 it, form the proper olfactory territory. 



Skull of Gavia ridibunda. 2nd stage. 



A somewhat more advanced stage (Plate XXVI. fig. 11) is shown in section as far as 

 to the cranio-facial isthmus (cfc) ; this figure supplies the deficiency of the last in not 

 showing a sectional view ; and it also shows some advance in the osseous deposits. 



This section has been made a little to the near side of the mid line, and shows the para- 

 sphenoid and basipterygoid and the whole interorbital septum uninjured. The back- 

 tilted large auditory capsule is seen to lie both on and in the occipital arch ; for the bony 

 matter creeping over the junction of the anterior and posterior semicircular canals is not, 

 in the bird, a large separate epiotic bone as in reptiles ; a rudiment appears afterwards 

 in this bird as in some others ; but at present the super- and exoccipital is largely walling 

 in the otic mass. 



The main periotic centre, the prootic (pro), is now a large oblong bone, and separated 

 by the unossified auditory crest from the alisphenoid (als), and by synchondrosis below 

 from the basisphenoid (bs) and the basioccipital (bo). The auditory nerve, or portio 

 mollis, pierces it, as also the portio dura, or facial nerve. Outside these foramina the 

 bone is scooped for the superincumbent brain, and becomes a mere crest ; the edge 

 itself is still soft. 



A clear wedge of bone is seen between the prootic and the exocciptal (eo) ; this is the 

 opisthotic (op) ; in the specimen it was becoming ankylosed to the prootic, but I have 

 diagram m atically given it a sharp suture from observation of the first stage. 



The large passage (8) between the exoccipital and the opisthotic is for the glosso- 

 pharyngeal and vagus nerves ; the anterior and posterior condyloid foramina (9) are seen 

 on the edge of the exoccipital. 



A large unossified space still exists on the crown of the arch of the anterior semi- 

 circular canal, and also in the recess for the cerebellar flosculus below it. If the bony 

 matter from the superoccipital had been somewhat less, then this space would have cor- 

 responded exactly to the region occupied by the epiotic bone in the Reptiles ; its rudi- 

 ment has not yet appeared in the Gull. 



The lozenge-shaped basioccipital is nearing the side piece (bo, eo) ; but there is a large 

 spheno-occipital synchondrosis, underfloored by the basitemporal plate (bt). 



From the great submucous bones, the parasphenoid proper, and the basitemporals 

 (pas, bt), an upgrowth of bone has taken place, metamorphosing the overlying chondro- 

 cranium. 



This bone, the basisphenoid (bs), underprops the optic nerves at their exit, and 

 embraces the dipping pituitary body, which passes down, as in osseous Eishes, to lie on 

 the submucous bony tract ; in the chondrocranium the pituitary space is open. 



Into this pouched space the internal carotid (ic) enters ; behind this space the cerebral 



