PLATANISTA. 513 



The squamous portion is nearly vertical, and the lower border of its rough 

 inner surface projects downwards and inwards; its base or under surface below this 

 border being deeply concave and marked with numerous foramina and forming a 

 cavity into which the outer wall of the petrous bone is received. The hinder 

 border of the squamous portion is marked by a fine ridge passing downwards, 

 outwards and forwards to the posterior margin of the base of the zygoma. 

 Behind this ridge, and projecting downwards and backwards from the squamous 

 portion and forming one-half of the upper boundary in young skulls, and in adults 

 the whole of the posterior wall of the external auditory meatus, is a nipple-shaped 

 postauditory process simulating a mastoid. To the inside of this process the 

 periotic and tympanic are firmly attached, and do not project behind its external 

 margin, although the posterior border of the rough surface of the tympanic, which 

 is attached to this process and the posterior fang of the periotic abut against the 

 anterior border of the exoccipital ; yet the fixedness of these two bones depends 

 entirely on the attachment to this process. Internally this plate presents a nearly 

 vertical pit into which the anterior fork of the mastoid is received and wholly 

 invested. The outer surface of the posterior wall of this pit has the second fang-like 

 process of the periotic applied to it. To the outside of the inner wall of the pit, the 

 rough surface ( vaginal ) of the tympanic is applied ; also investing the posterior 

 fang of the mastoid and the styloid process. The posterior border of this process 

 articulates with the exoccipital, to which it only becomes united in very old skulls. 

 The lower border of the parietal surface of the squamosal, which is at a considerably 

 higher level than the pit, projects inwards as a thin plate with the upper internal 

 border of the pit, constituting a broad longitudinal furrow, in which lies the anterior 

 division of the periotic. 



The zygomatic process of this skull, as is well known, is remarkably large 

 and has a considerable outward curve. It is very deep at its base, but its anterior 

 extremity is narrowed to one-half of the vertical height of the former part and 

 is applied to the frontal forming the posterior border of the orbit. The superior 

 border is nearly longitudinal, with only a very slight upward tendency, while 

 the lower convex border has a decided upward and forward curve. The external 

 surface is markedly convex and the internal, deeply concave ; so much so, that 

 the powerful-looking bone has no commensurate thickness and is occasionally 

 fractured. At its base above there is a groove for the temporal muscle. The pos- 

 terior basal margin is concave, and at its upper end, in adults, it forms an arch over 

 the external auditory meatus, springing from the anterior surface of the post- 

 auditory process ; while, in young skulls, this process forms the whole of the upper 

 external boundary of that opening. Its lower end is directed backwards and 

 inwards, terminating in a rough postglenoid border, from which a thin, inwardly pro- 

 jecting plate is prolonged upwards and forwards in a curved manner to the anterior 

 root of the petro-ptery go-tympanic plate. This glenoid plate constitutes part of the 

 floor and of the external wall of the deep fossa that occurs on the inner surface of 

 the base of the zygoma. On the inner wall of this fossa, between the periotic and 



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