AND DEVELOPMENT OF THE BIRD'S SKULL. 143 



Outside these spaces the bone is ear-shaped, the substance of it spreading towards 

 the ear-capsule. 



These capsules are very large, and being impacted into or, at least, grafted upon the 

 large occipital arch and wall, appear very considerably in the back of the head. 



Ossification has appeared in them, and the largest bony piece, the prootic (pro), is seen 

 behind the quadrate (q) in the lateral view (fig. 1) ; the other centres will be described in 

 more advanced stages, but the opisthotic is already well developed, although much 

 smaller than the prootic ; the epiotic has not yet appeared. 



The common sphenoidal region is at this stage a part full of interest to the mor- 

 phologist. The three divisions of the great winged parasphenoid, although ankylosed, are 

 very evident. They are the rostral portion in front (figs. 1 & 2, pas), and the basi- 

 temporal plates (bt) behind. These three bones are setting up ossification in the 

 overlying chondrocranium, although developed at first as parostoses and quite inde- 

 pendent of it. 



As in the Ichthyopsida, these primordial splints are very large in relation to the rest 

 of the skull. That this should be so, shows how little the Bird is related to the ordinary 

 Lacertian type of Reptile, where the only remnant (not always present) of the para- 

 sphenoid is the anterior part. On each side of the scooped (Eustachian) part of the 

 median bone there is a cartilaginous bud; this is the basipterygoid process (bpg), a 

 typically Pluvialine structure. 



Behind these knobs the bone spreads into large wings that enclose the " anterior 

 tympanic recesses." Beneath these imperfect trumpets lie the basitemporal bones (bt) ; 

 they have nearly melted into each other at the mid line, and above they are being 

 soldered to the tympanic wings of the parasphenoid. 



But the overlying cartilage around the pituitary body and beneath the optic nerve (2) 

 has become osseous directly from the bone beneath ; this is the true basisphenoid (bs) ; 

 the square inturned large alisphenoids (als) are also ossified. 



A large bony leaf is seen in front of the orbital septum ; it is trifoliate behind, and does 

 not nearly reach the membranous window between the eyes (iof). 



In the figure (fig. 1) the septal portion of the nasal labyrinth has been cut away (see 

 this part from below in fig. 7) ; this is at the cranio-facial " notch," and the part in front 

 is only supplied by the orbito-nasal branch of the fifth nerve, and not by the olfactory. 



This bony plate is the perpendicular or mesethmoid (pe) ; its huge size has relation to 

 the abortive development of the anterior sphenoid, and its bony matter will not cease to 

 grow until it has reached that of the posterior sphenoid (bs) beneath the optic nerve (2). 



Nothing but development could possibly throw the least light upon this anomaly ; 

 there are no orbito-sphenoidal alse, and the presphenoid is merely that bar of cartilage, 

 afterwards bone, which is seen bounding the posterior half of the interorbital fenestra 

 (iof) behind ; it reaches neither the top nor the bottom of the interorbital chondro- 

 cranium. 



The top of the chondrocranium finishes behind in a median projection, behind and 

 between the upper turbinal scrolls (aliethmoids) ; this is a rudiment of that cartilaginous 



