TYLOTOTEITON. 855 



internal margins of the axe-like heads of the two bones are widely separated from 

 each other anteriorly for the reception of the cartilage. Behind this, they are 

 nearly in contact for a short way, hut they become widely divergent posteriorly. 



The posterior external border of the bone is marked by a deep notch, which 

 defines the internal border of the inner nostril. The internal margin of each plate 

 is roughly serrated, the serrations corresponding to the positions of the palatine 

 teeth. The divergent portions of the plate lie on and between the parasphenoid 

 and orbito-sphenoid and on the former as far as the pterygoid. 



The frontal (figs. 29 a and 29 b) is by far the largest bone of the skull, and 

 articulates with the nasals, prefronto-lachrymals, temporal, parietals, and orbito- 

 sphenoids. It consists of two portions, a flattened body and an appended 

 orbital process which enters into the middle of the upper wall of the orbit. The 

 body is slightly convex externally, and the frontal suture carries one-half of the 

 central cranial crest. The orbital process is covered by a rough osseous surface, 

 that part of the lateral cranial crest which intervenes between the prefronto- 

 lachrymal and the squamosal, and it forms the external margin of the temporal 

 fossa. 



The parietal (figs. 30 a and 30 b) is an almost square bone and very much smaller 

 than the frontal, with its posterior external angle curved upwards to rest on the 

 temporal and arch of the exoccipitals. The middle is occupied by the median 

 cranial crest, which begins near the posterior extremity of the bone, as a prominent 

 raised ridge. 



The exoccipital and auditory capsule (figs. 31(%, 315, and 31c) consist of one solid 

 bone, which forms a large chamber for the organ of hearing and lodges nearly one-half 

 of the brain. It presents, however, in its fenestra ovalis, semicircular canals and 

 condyles, a series of prominent landmarks which enable it to be mapped out. 

 It may be described as consisting of two portions, one embracing the exoccipital 

 condyle and an arched process immediately anterior to that articular surface 

 forming the lateral and upper wall of the brain cavity, posterior to the parietals, 

 and another, the greater of the two, in which the auditory chamber and its parts 

 are lodged. 



These two portions are defined thus : a ridge, with a groove on its under side, 

 begins on the superior anterior margin of the fenestra ovalis, and is prolonged 

 onwards and forwards over the condyle and along the posterior margin of the 

 arched process of the exoccipital to its tip, from whence it can be traced along 

 its anterior margin, as a faint line. Another ridge springs from the inferior, 

 anterior border of the fenestra ovalis and runs forwards and inwards, nearly to the 

 foi-amen magnum, and then outwards and forwards on the under surface to 

 almost the anterior termination of the cranial floor. It is true that the latter ridge 

 defines the surface of the cranial floor covered by the parasphenoid ; and as this 

 area, from its relations to the condyles, may be regarded as exoccipital, may not 

 all the remaining portion of the capsule external to it and entering into the forma- 

 tion of the floor of the auditory chamber be considered as the opposite element, 



