1899.] 



CRANIAL OSTEOLOGY OF THE PAEEOTS. 



13 



posterior, below and in front into the long compressed or pointed 

 anterior tympanic recess. A bar of bone, part of the prootic, runs 

 forwards near the middle of the cavity, boiiuding the lower border 

 of the superior recess, and bearing anteriorly the articular surface 

 for the inner head of the quadrate bone, immediately below which 

 is the tiny orifice of the canal for the external ophthalmic artery. 

 Into the posterior recess, below the fenestrje ovalis and rotunda, 

 opens from behind the large aperture which transmits the so- 

 called tensor tympani muscle, and the recess itself runs backwards 

 and downwards externally to the orifice, within the so-called 

 paroccipital process. 



In the Grey Parrot the tympanic orifice is moderately wide : 

 looked at from a little to the front it is very nearly semicircular ; 

 from a little way behind it appears crescentic, from the manner 

 in which the slightly curving border of the posterior or exoccipital 

 wall encroaches on the front of the cavity. The nearly straight 

 ■but slightly curving posterior border, the somewhat angular notch 

 above, and the more pointed notch below, that are visible in the 

 figure, are the chief points that catch the eye. We shall find that 

 the shape of the tympanic orifice and the extent to which the cavity 

 is walled in differ much in the different genera, and chielly in 

 relation to the extent of development of the posterior wall ; and 

 that we have great concurrent variation in the area between the 

 auditory meatus and the descending occipital ridge. In some cases, 

 in correspondence with the shape of the quadrate bone, the glenoid 

 cavity for its inner head will be found widely separate, in others 

 scarcely separate or not at all, from the squamosal facet for its 

 outer one. And again the relative dimensions of the recess will 

 be found to vary, the posterior one in particular being sometimes 

 very greatly reduced. 



rig. 2. 



oh. 



.C - 



Quadrate bone of Psittacus erithacus. 



iM., o.h., inner and outer heads; sk., shaft; a.p., anterior process; J.c, jiigal 

 cup; pt.c, pterygoid condyle, distinct from and in front of the long 

 mandibular condyle. 



The quadrate bone (fig. 2) shows us a long, straight, slender 

 sJtaft, and a flattened body, whose lower margin, almost circular 

 in contour, forms the elongated simple articular surface, playing in 



