388 THE VERTEBRATE SKELETON. 



surface a pterygoid plate, which is united in front with the 

 palatine, and below with the pterygoid. The alisphenoids are 

 united above with a pair of large nearly square bones, the 

 parietals (fig. 73, 2), which meet one another in the mid-dorsal 

 line. The line of junction is frequently drawn out into a strong 

 ridge, which forms the anterior part of the sagittal crest. 



The frontal segment, which surrounds the anterior part 

 of the brain, is closely connected along almost its whole 

 posterior border with the parietal segment. 



Its base is formed by the presphenoid (fig. 75, 12), a 

 very deep unpaired bone, narrow and compressed ventrally, 

 and with an irregular dorsal surface. The presphenoid is 

 continuous with a second pair of wing-like bones, the orbito- 

 sphenoids. Each orbitosphenoid meets the alisphenoid 

 behind, but the relations of the parts in this region are 

 somewhat obscured by a number of large foramina piercing 

 the bones, and also by an irregular vacuity, the foramen 

 lacerum anterius or sphenoidal fissure, which lies between 

 the orbitosphenoid and alisphenoid, separating the lateral parts 

 of the parietal and frontal segments, in the same way as the 

 space occupied by the auditory bones separates the lateral 

 parts of the occipital and parietal segments. The orbito- 

 sphenoids pass obliquely forwards and upwards, and are united 

 above with a second pair of large membrane bones, thefrontals 

 (fig. 73, 3). The outer side of each frontal is drawn out into 

 a rather prominent rounded postorbital process (fig. 73, 10), 

 from which a ridge converges backwards to meet the sagittal 

 crest. The anterior part of the frontal is produced to form 

 the long nasal process, which is wedged in between the nasal 

 and maxilla. 



The cranial cavity is continuous in front with the nasal or 

 olfactory cavities, but the passage is partially closed by a 

 screen of bone, the cribriform plate (fig. 72, 5), which is 

 placed obliquely across the anterior end of the cranial cavity, 

 and is perforated by a number of holes through which the 



