49« MANUAL OF ZOOLOGY sect. 



in position, which forms the floor of the most posterior part 

 of the cranial cavity ; it bears the lower third of the occipital 

 condyles. Articulating in front with the basi-occipital is a 

 plate of bone, also horizontal in position, which forms the 

 middle part of the floor of the cranial cavity. This is the 

 basi-sphenoid ' ; on its upper surface is a depression, the sella 

 tunica or pituitary fossa, in which the pituitary body rests. 

 In front of it is another median bone of laterally compressed 

 form, the pre-sphenoid. Connected laterally with the basi- 

 sphenoid and pre-sphenoid are two pairs of thin irregular 

 plates, the ali-sphenoid (as) behind and the orbito-sphenoid 

 (o. spK) in front. The ali-sphenoids are broad wing-like 

 bones, each produced below into a bilaminate process, the 

 pterygoid process. 



The boundary of the anterior part of the brain case is 

 completed by a narrow plate of bone, the cribriform plate 

 of the ethmoid, perforated by numerous small foramina for 

 the passage of the olfactory nerves. This cribriform plate 

 forms a part of a median vertical bone, the mesethmoid, the 

 remainder of which, or lamina perpendicularis, forms the bony 

 part of the partition (completed by cartilage in the unma- 

 cerated skull) between the nasal cavities. Fused with the 

 mesethmoid are two lateral, thin-twisted bones, the ethmo- 

 turbinals, and with its inferior edge articulates a long median 

 bone with a pair of delicate lateral wings, the vomer. Roof- 

 ing over the part of the cranial cavity, the walls and floors of 

 which are formed by the sphenoid elements, is a pair of 

 membrane bones, the parietals (pa), and further forward 

 another pair, the fron tals (fr). Between the supra-occipital 

 and the parietals is a median ossification, the inter-parietal 

 (int. pa). Below the parietal and frontal is a broad bone (sq), 

 the superior margin of which is bevelled off. This is the 

 squamosal. It gives off in front a strong zygomatic process, 



