278 MR. W. K. PARKER ON THE OSTEOLOGY OF BAL^NICEPS REX. 



day after hatching, these three pairs of osseous centres are about equal to what may be 

 seen in the Chick on the eleventh day (nine days before hatching). 



Looking at the side of the Balseniceps' skull between the post-frontal and squamosal, 

 there may be seen, eight lines mesiad of the anterior spur of the latter, a large oval 

 foramen, which has a conjugational relation between the petrosal, ali-sphenoid, and 

 basi-sphenoid. This passage (the foramen ovale) transmits the chief part of the tri- 

 geminal nerve, and is one of the best landmarks in the study of the anatomy of the 

 cranium in all the Vertebrata. This aperture is wide below and narrow above ; it is four 

 lines in extent in its long diameter, the direction of which is backwards and outwards. 

 A bristle passed across the floor of the cranium through both the foramina ovalia lies 

 one line behind the posterior clinoid processes. 



Ali-sphenoids. (PI. LXV. fig. 1, as.) 



The ali-sphenoid of Balaeniceps is crossed at its upper third by an ascending crescentic 

 ridge which joins the post-frontal process and forms the antero-inferior boundary of the 

 temporal fossa. At one-eighth of an inch in front of the foramen ovale there is a tri- 

 angular eminence in a line with the middle of that passage ; and at the same distance 

 in front of this eminence, but higher up, a twin passage exists. This divided passage 

 is a little below and behind the great optic foramen, which opens freely into its fellow of 

 the opposite side, and which notches the posterior border of the connate orbito-sphenoid. 

 The optic foramen of Balaeniceps is half an inch in front of the foramen ovale and one- 

 third of an inch higher up. A bristle passed through both the optic foramina lies in front 

 of the anterior margin of the very deep sella turcica, and passes across the middle of 

 the anterior or, rather, internal margin of the ali-sphenoids. The optic foramen is four 

 lines in diameter; its shape is ovoid, the narrow end pointing outwards and a httle 

 downwards. Half an inch above this foramen another exists, half its size ; this is the 

 remains of that membranous tract in the primordial cranium between the cartilaginous 

 interorbital septum and the cartilage in which the ali-sphenoid was developed : through 

 a passage in the lowest part of this tract the optic nerve passed. By means of the 

 extension of ossific matter from the growing ali-sphenoid, and the diverging alse of the 

 orbito-sphenoid, this membranous tract becomes bony for more than its middle half, its 

 upper part continuing membranous, and its lower third becoming the large well-mar- 

 gined optic foramen. 



There is a considerable eminence on the ali-sphenoid, two lines above the foramen 

 ovale, which breaks into two elegant crescentic ridges, the anterior ridge passing at the 

 distance of a line behind the one already described to the base of the post-frontal, whilst 

 the posterior ridge passes outwards and backwards to the quadrato-squamosal joint. 

 The bone across this middle part is very thick and cellular. 



