216 THE SKULL. [CHAP. 



The periotic is a rounded, very dense bone, characterised 

 as usual by having on its inner or cerebral side the large 

 meatus auditorius internus, and on the surface turned 

 towards the tympanic cavity, the two fenestrae (ovalis and 

 rotunda). The tympanic and periotic bones are separated 

 along their inner margin by a narrow fissure, the " tympano- 

 periotic fissure " ; but they are united externally in front of 

 the external meatus auditorius, and more firmly posteriorly, 

 where a tongue- shaped process (Per) projects backwards 

 and outwards, fitting into a groove formed by the junction 

 of the squamosal and exoccipital, and which is the principal 

 point of attachment of the tympano-periotic bone to the 

 rest of the skull. This process resembles in its relations the 

 mastoid of ordinary Mammals, but in young Cetaceans it 

 may be seen to be composed of two nearly equal parts, in 

 close apposition with each, the inferior being derived from 

 the tympanic, and the superior from the periotic, so that 

 the latter alone can represent the "pars mastoidea " of 

 other Mammals. 



The mandible (Fig. 65) consists of a pair of nearly straight 

 compressed rami, wide behind and gradually narrowing to 

 the symphysis (.$), where they usually become ankylosed in 

 adult animals. The condylar articular surface (cd) is small 

 and looks almost directly backwards, being placed on the 

 hinder edge of the ramus. The coronoid process (c/) is 

 quite rudimentary. The angle is square and flat. The 

 entrance to the dental foramen on the inner side is ex- 

 tremely wide and infundibuliform. 



The ossified portion of the hyoid in the adult Dolphin 

 consists of a large subcylindrical, slightly curved stylohyal (s/i) 

 on each side, and a flattened crescentic median bone, com- 

 posed of the ankylosed basihyal (bit) and thyrohyals (///). The 

 stylohyal is connected above by a slender cartilaginous rod 



