514 CETACEA. 



tympanic, there is a small zig-zag fissure immediately external to the processus 

 gracilis of the malleus, viz., Glasserian fissure. The articular siirface for the man- 

 dible is an oval concavity looking forwards, inwards and downwards. The petro- 

 pterygo-tympanic is a comparatively thin plate arising by two roots, contracted at 

 its origin, but expanding at its extremity into three prongs in youngish specimens ; 

 whereas in adult skulls the extremity is more or less rounded with a central 

 arched strong spine-like process, which in yoimg skulls is the more anterior of the 

 three prongs. The anterior root is situated at the upper border of the zygoma at 

 its union with the parietal plate of the squamosal, but on its inner aspect it is con- 

 tmuous with the upward ridge from the glenoid. The posterior root, which is the 

 largest, forms the roof of the postglenoid fossa, and lying longitudinally below 

 the parietal surface of the squamosal has its posterior extremity slightly nodose, 

 and on a line with the posterior margin of the base of the zygoma, and with a deep 

 hollow behind it, into which the periotic is received. The inner surface of this 

 pterygoperiotic plate is applied over the rough external surface of the pterygoid, 

 and its terminal spine arches over the orifice through which the inferior maxillary 

 nerve makes its exit from the cranium ; being also applied to a small rough surface 

 on the free margin of the external orifice of the foramen ovale, dividing that 

 foramen, Avhich is large, into two halves, a superior and inferior. The posterior 

 surface of the plate closely invests the anterior and external borders of the 

 periotic, and is applied more or less to the upper concave border of the long 

 Eustachian process of the tympanic. Its upper border is applied against the 

 pterygoid, which at tliis point overlaps the anterior division of the wing of the 

 basisphenoid. The ridge I have described constitutes the lower boundary of the 

 very large temporal fossa, and the surface of the plate below it is smooth but 

 slightly concave. 



Periodic (PL XL, fig. 9, jje). — In position it presents two sui'faces and three 

 borders ; the former look upwards and downwards, and the latter are external, 

 internal, and posterior. The internal border has a deep and wide notch before the 

 cochlea, prolonged as a deep furrow on the nnder surface, thus resolving this aspect 

 of the bone iato two well-marked portions, an anterior and posterior. On the upper 

 surface this notch is prolonged outwards and backwards along the anterior border of 

 the cochlea, so that the anterior division of the bone in this aspect is more extensive 

 than on the opposite face. The anterior portion is a powerful inwardly curved 

 hook, convex from before backwards, presenting above on its inner border an 

 upwardly projecting sharp conical, sometimes style-like process, that fits into a 

 vacuity between the posterior and superior extremities of the internal and external 

 plates of the pterygoid. The inner side of this process is marked by a groove which 

 passes downwards and backwards to a very minute foramen on the inner border of 

 the great notch at the anterior border of the cochlea ; the commencement of a 

 canal that opens into the aqueduct of Eallopius near its termination and doubtless 

 conveys the chorda tympanic nerve. Internal to this foramen on the outer surface 

 of the periotic, there is another small foramen leading to a nutriment canal which 



