304 Mr. D. M. S. Watson on 



The front part of the basisphenoid has been described as 

 an exceedingly thin but deep plate ; somewhat further 

 forward this plate swells out and forms two powerful pro- 

 cesses ; its lower pai-t is rather broader posteriorly^ but in 

 this region contracts so as to form a narrow ridge risiuij 

 above the table formed by these processes (Hgs. 3 & 6). 



These processes are tlattened and at least 1 cm. long ; 

 they give attachment to the pterygoids, and from their outer 

 and posterior corners send back long slender splints which 

 lie on the inner sides of the posterior rami of the pterygoids. 



Anteriorly the thin ridge on the lower surface is continued 

 into the vomer, which has a thin dorsal ridge almost certainly 

 directly continuous with the thin part of the basisphenoid 

 in advance of the pituitary fossa; there is a gap of 4 mm. in 

 which this ridge is not exposed. 



Pterygoid. — The pterygoid is a large triradiate bone which 

 is separated from its fellow of the opposite side by the vomer 

 and basisphenoid, and is conveniently divided into two parts 

 for descriptive purposes, the part behind the basisphenoidal 

 articulation and that before (tigs. 3, 5, & 6). 



The posterior part consists of the posterior ramus and the 

 ascending plate whi:h forms the columella cranii (tig. 5). 



The posterior ramus is a very thin (about 5 mm.) plate of 

 bone extending back parallel to the antero-lateral face of 

 the combined basisphenoid and periotic ; it is supported 

 anteriorly by the posterior process of the buttress of the 

 basisphenoid, and further back by the process of the periotic 

 which has already been described (fig. 3). 



Laterally this bone passes out to the quadrate, and is sepa- 

 rated from the paroccipital process by a thin inwardly 

 directed process of the squamosal. This ramus forms the 

 anterior wall of the tympanic cavity, the roof of which is 

 pierced by the large pterygo-paroccipital foramen and by the 

 deep fossa lying between tlie posterior ramus of the pterygoid, 

 the pterygoid process of the basisphenoid, and the body of 

 that bone; this fossa opens directly into the brain-cavity 

 both laterally through the pituitary fossa and dorsally ; at 

 its posterior end its outer wall is pierced by the foramen for 

 the V.^ nerve. 



Rising directly from the upper edge of the posterior ramus 

 is the ascending plate, a broad thin lamina of bone articu- 

 lating above with the parietal ; it is slightly damaged by 



a 



vertical crushing in the specimen (fig. 5). The upper border 

 of this plate is almost straight and of great length. The 

 anterior border has a deep wide notch near its lower edge, 



