156 GENERAL OBNITHOLOGY. 



whole forming an outer lateral nul frotn (juadiate to jiremaxillary, like a duplicate of the 

 pterygo-palatiue lod from the same t(j the sauie. 



Among occurrences of later stages are to be noted the development in membrane in the 

 middle line below of the vomer, borne upon the end of the rostrum ; the roofing in of the 

 whole skull by the parietal, squamosal, frontal and nasal bones ; the completion of the periotic 

 bones as the prootic, epiotic and opisthotic, which form the otic capsule; the development of 

 lacrymal hones, bounding the orbits of the eyes in front. Absorption of the ndddle wall of 

 cartilage between the nasal and orbital cavities nicks off the nose parts from those of the orbit 

 (fig. 70, between nth and eth) ; and certain changes in the orbital septum develop the orhito- 

 sphenoids. Very nearly all the bones of a bird's skull having thus been accounted for, we may 

 next consider them in their adult condition. Eeference shcjnld now be made to figs. 62. 

 63, 70, 71. 



The Occipital Bone (fig. 62, 70, 71) forms the back part of the floor of the skull, and lower 

 part of the back wall of the skull ; neither its boundaries nor its composition is visible in 

 adult skuUs. It is formed by the basioccipital, bo, below in the middle line ; the supra-occipital 

 so, above in the middle line ; the exoccipital, eo, on either side. These bound the foramen 

 magnum (fig. 69, fni), where the nerve mass makes its exit from the cavity of the cranium into 

 the tube of the spinal column. At the lower part of the foramen is the protuberant occipital 

 condyle (figs. G8, 71, oc), borne chiefly upon the basioccipital, but to the formation of which the 

 exoccipitals also contribute; the latter flare widely on each side, into the tympanic vidngs, which 

 bound the external auditory meatus behind. The true basioccipital is mostly covered by the 

 underlying secondary bone, the hasitemporal (69, 70, ht), which extends fi-om one tympanic 

 cavity to the other, and more or less forward in the middle line to the sphenoidal rostrum. 

 Openings to be observed in the occipital region, besides the great foramen, are those for the 

 hypoglossal nerve, 9, near the condyle ; for the parts of the vagus nerve, 8, more laterally, and 

 the carotid canal, ic: also, above the foramen magnum, openings for veins, sometimes of great 

 size, as in fig. 63, j. 



The Parietals (figs. 62, and 70, jp, 71)- — Proceeding up over the brain-box, the next 

 bones are a pair of parietals, between the occipital behind, the frontal before, and the squa- 

 mosal beside ; but their limits are rarely if ever to be seen in adult skulls. They are relatively 

 small in birds ; simply squarish plates, bounded as said, coming together in the midline. 



The Frontals (fig. 62, and 70,/, 71), originally paired, soon fuse together, and with sur- 

 rounding bones of the skull, though maintaining some distinction from those of the nose and jaw. 

 These roof over much of the brain cavity, close in much of it in front, and form the roof and 

 eaves of the great orbital sockets. Anteriorly in the middle of the forehead line the feet of the 

 nasal process of the premaxillary are implanted upon the frontal, usually distinctly ; more 

 laterally, the nasal bones are articulated or anchylosed ; this fronto-naso-premaxillary suture 

 forming the fronto-facial hinge, (fig. 63, x) by the elasticity or articulatit'iu of which the upper 

 jaw moves upon the skull, when acted on by the palatal and jugal bars. In the midst of the fore- 

 head the two halves of the frontal sometimes separate, as they do in the fowl, allowing a little 

 of the mesethmoid to come to the front. In the middle line, underneath, the frontals fuse with 

 whatever extent there may be of the mesethmoid which forms the lengthwise inter-orbital 

 septum, and often a crosswise partition between the orbital and nasal cavities. To the antero- 

 external corners of the frontal are articulated or anchylosed the lacrymals. The post-frontal 

 process.^ morphologically the post-frontal or sphenotic bone, bounds the rim of the orbit behind ; 



1 There is apparently some ambiguity in tlie use of the term " post-frontal " process by different authors. It 

 would appear that this process, bounding the rim of the orbit behind, may be a projection of the frontal bone, and 

 therefore strictly a post-frontal process. Or that, as said by Owen for Rhea, it may be a separate bone, and there- 



