xil PHYLUM CHORDATA 493 



of bone above the foramen magnum. The exoccipitals Heat 

 the sides of the opening, and each bears the greater part of 

 the somewhat oval prominence or condyle with which the 

 corresponding surface of the atlas or first vertebra articulates. 

 The basiocdpital (b. oc.) is a median plate of bone, almost 

 horizontal 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 

 basioccipital is a plate of bone, also horizontal in position, 

 which forms the middle part of the floor of the cranial 

 cavity. This is the basisphenoid (b. sph.) ; on its upper 

 surface is a depression, the sella turcica, or pituitary fossa, 

 in which the pituitary body rests. In front of it is another 

 median bone of laterally compressed form, the presphenoid. 

 Connected laterally with the basisphenoid and presphenoid 

 are two pairs of thin irregular plates, the alisphenoid (as.) 

 behind and the orbitosphenoid (o. sph.) 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 unmacer- 

 ated 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 (vo.). 

 Roofing 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 



