152 ON THE MORPHOLOGICAL CONSTITUTION OF 



Before dismissing the consideration of this important cen- 

 trum in the bird, I would direct attention to certain interesting 

 modifications which it may undergo. In the first place, it 

 may, like many other bones in the cranium of the bird, become 

 greatly dilated and altered in form by the development of 

 air-cells in its interior. The pneumatic openings are two in 

 number, one on each side of the anterior margin below the 

 superior horizontal plate. The pneumatic excavation and 

 dilatation extends backwards more or less in certain species ; 

 and in some owls the bone presents the form of a cubical 

 cellular mass. This peculiarity of form might be adduced in 

 support of Professor Owen's doctrine of the formation of this 

 bone from the coalescence of the pre-frontals ; but then it 

 will be observed that the increased breadth of the bone is not 

 due to incomplete mesial fusion of lateral parts, but to ex- 

 pansion from the mesial plane, for the olfactory nerves still 

 run forwards in grooves on its lateral aspects, although these 

 may be deep in front, and posteriorly their margins may 

 overlap the nerves. The expansion of the pre-sphenoidal 

 centrum also produces a remarkable separation of the optic 

 foramina. As explanatory of this effect, I would observe, 

 that the development of this bone in the chick shows that it 

 forms the posterior border of the common optic foramen by 

 means of a pair of processes which project from its posterior 

 inferior angle like the limbs of the letter Y. When, therefore, 

 the bone takes on transverse dimension, the single optic chasm 

 separates into two optic foramina, which, in Strix flammea 

 are three-eighths of an inch asunder. 



The separation of the optic foramina from the pneumatic 

 expansion of the pre-sphenoidal centrum leads me, in the 

 second place, to observe, that the characteristic separation of 

 these orifices in the extinct forms Dodo, Dinornis, Palapteryx, 

 did not depend entirely on pneumatic expansion of the pre- 

 sphenoidal centrum, nor on such width of that bone as might 



