192 THE SKULL. [chap. 



towards the tympanic cavity the fenestra ovalis and rotunda. 

 These two 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 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. 6i) consists of a pair of nearly straight 

 compressed rami, wide behind and gradually narrowing to 

 the symphysis (j-), 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 {cp) 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 pordon of the hyoid in the true Dolphins 

 consists of a large subcylindrical, slightly curved stylohyal on 

 each side, and a flattened crescentic median bone, composed 

 of the ankylosed basihyal and thyrohyals. The stylohyal is 

 connected above by a slender cartilaginous rod to a small 

 ossified tympanohyal, which becomes ankylosed to the 

 periotic in the usual situation, close to the stylomastoid 

 foramen ; it has also a strong ligamentous attachment to 

 the prominent rough paroccipital process of the exoccipital. 



