OSTEOLOGY OF BIRDS 



115 



above, with a shallow median groove present, which, posteriorly, 

 passes between the parietal eminences. A lacrymal bone has its 

 supraorbital part markedly reduced in size, while the broader and 

 backward curving portion below, in the older individuals, fuses with 

 the external margin of the somewhat vertically narrow pars plana. 

 Regarding this skull next upon its lateral aspect, we are to note 

 that the squamosal process is very blunt and smooth, the post- 

 frontal one is a much better produced spine, and the valley between 

 them rather wide, although the' crotaphyte fossa is not particularly 

 extensive. 



Either aural entrance is capaci- 

 ous, flaring, and markedly open, 

 and, for the moment, carrying this 

 examination to the base of the 

 cranium we are surprised to find 

 that not only are the anterior walls 

 of the Eustachian tubes open and 

 nonossified, but the anteromedian 

 entrance to these channels is en- 

 tirely sealed up. Moreover, the 

 tube appears to be double, and both 

 being extended to the osseous aural 

 cavity, and both opening anteriorly, 

 one just above the other, external 

 to the base of the basisphenoidal 

 rostrum. Foramen ovale often is 

 double ; foramen rotundum is dis- 

 tinct, as are the smaller nervous 

 foramina at its outer side. The 

 foramen for the passage of the first 

 pair is small (just large enough to 

 pass the nerve). They are double 



in either orbit, and open upon a completely exposed twin 

 groove, which transmits the nasal vessels and nerves to the 

 rhinal chamber and orbital roof. A large subcircular vacuity 

 occupies the center of the interorbital septum; and a large 

 irregular opening occurs among the lacrymal, frontal, and pars 

 plana. Another is seen, within the orbit, on the anterior wall of 

 the brain case. In front, the anterior border or edge of the 

 mesethmoid extends .beyond the common plane of the ethmoidal 

 alae; and, above, this border always fails to meet the ossified sep- 



Fig. 56 The skull of Pandion seen 

 upon superior view. Natural size 



