230 GENERAL ORNITHOLOGY part ii 



formed ; the condyle is perfect ; the foramen magnum is circum- 

 scribed by the ex- and supra-occipitals, eo and so, Fig. 6 9. Investing 

 bones, formed in membrane without previous cartilage, are becom- 

 ing apparent. The basitemporal, M, and parasphenoid, rbs, are en- 

 grafting upon the base of the skull. The prenasal cartilage, pn, now 

 at its full growth, is beginning to decline ; on each side of it is 

 formed a three-forked bone, the premaxillary, px, having superiorly 

 nasal, and laterally palatal and dentary processes. This bone is to 

 grow to great size, forming most of the upper beak, and starving 

 out the maxillary, which in mammals is the principal bone of the 

 upper jaw. The palatal, pa, and pterygoid, pg, bones are ossified, 

 and the quadrate, q, is ossifying. Between the premaxillary and 

 the quadrate are the bones forming the zygoma, or jugal bar, de- 

 veloped in the outer part of the maxillopalatine bar of the earlier 

 embryo. They are the weak maxillary, mx, with its ingrowing 

 process, the maxillopalatine bone, mv^p ; next the jugal, j.; then the 

 quadratqjugal, gp ; the whole forming an outer lateral rod from 

 quadrate to premaxillary, like a duplicate of the pterygopalatine 

 rod from the same to the same. 



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, 

 or otocrane ; the development of lacrymal hones, bounding the orbits of 

 the eyes in front. Absorption of the middle wall of cartilage be- 

 tween 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 orbitosphenoids. Very nearly all 

 the bones of a bird's skull having thus been accounted for, we may 

 next consider them in their adult condition. Reference should 

 be made to Figs. 62, 63, 70, 71. 



The Oeeipital Bone (Figs. 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 are visible in adult skulls. 

 It is formed by the hasioccipital, ho, below in the middle line ; the 

 supraoccipital, so, above in the middle line ; the exoccipital, eo, on either 

 side. These bound the foramen magnum (Fig. 69, fm), 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. 68, 71, oc), borne chiefly upon 

 the hasioccipital, but to the formation of which the exoccipitals also 

 contribute ; the latter flare widely on each side, into the tympanic 

 wings, which bound the external auditory meatus behind. The true 

 hasioccipital is mostly covered by the underlying secondary bone, 



