272 on the structure of the skull. 



in by the squamosal. Between these bones there is left, at the 

 apex of the chamber, a considerable irregular aperture, which 

 communicates with the cranial cavity. 



The anterior and outer part of the under-surface of the 

 squamosal is produced downwards into a great trihedral pillar, 

 the obliquely truncated inferior face of which bears the articular 

 surface for the mandible (GI, Fig. 107). Behind this the squa- 

 mosal presents a comparatively low wedge-shaped ridge (a, 

 Fig. 108), between which and the "trihedral pillar" is a groove; 

 Fio . 108 while behind it, or between it and 



the ex-occipital, there is a deeper 

 and wider transverse channel. 



The periotic bone is irregu- 

 larly triangular ; the apex of the 

 triangle, turned inwards and for- 

 wards, is thick and rounded, the 

 anterior, posterior, and outer edges 

 being thinner and more or less 

 O/iO irregular. The upper smooth and 

 ^\ concavo-convex surface of the 



Fig. 108.— Enlarged view of the chamber periotic adjusts itself to the Under- 



which lodges the left tympano-periotic c r> ,i -i -i ., 



bone of the fetal Baimia i austral, is.- surface ol the squamosal, where it 

 a, the « wedge-shaped ridge " of the forms the roof of the funnel-shaped 



squamosal ; C, the aperture which . rT11 _ 1 . . 



leads into the interior of the skull; Cavity. Hie apex Ol the penotlC, 



f.o, fenestra rotunda. however, projects beyond this, and 



incompletely divides the irregular aperture above mentioned 

 (b, Fig. 108) into an anterior division, which corresponds with 

 the foramen ovale and foramen lacerum medium, and a poste- 

 rior which answers to a foramen lacerum poster 'his. 



The under-surface of the periotic, much more irregular, is 

 divisible into three regions : an outer anterior ; an outer pos- 

 terior ; an internal. The first and second are separated by a 

 deep triangular notch in the outer margin of the bone, into 

 which the inner end of the wedge-shaped ridge of the squamosal 

 is received. The first, broad and short (PrO), presents a rough 

 surface in front, with which the tympanic articulates, and 

 eventually anchyloses; and behind, a concave surface, which, 

 entering into the roof of the tympanic cavity, answers to the 



