SKULL MAMMALS 197 



usually fuses with the petrosal, often also with the squamosal and 

 sometimes with the alisphenoid. At the place where the Meckelian 

 passes from the malleus to the lower jaw, a petrotympanic (Glaserian) 

 fissure persists between tympanic and petrosal, this being wide in a 

 few scattered genera. In many mammals (especially Carnivores and 

 Cetacea) the tympanic expands to a vesicle (tympanic bulla, fig. 205) 

 surrounding the outer part of the tympanic cavity and auditory 

 meatus, this being supplemented in some genera by an entotympanic 

 (metatympanic) bone of cartilage origin, which sometimes retains 

 its individuality. 



The tympanic bone arises in the same relation to Meckel's cartilage as does 

 the angulare and seems to be its homologue, but it has lost its connexion with the 

 rest of the lower jaw along with the shifting of the hinge from the quadrate to 

 the squamosal, and, like the quadrate, has become accessory to heanng. 



Marsupials have an auditory bulla formed from the alisphenoid. The ento- 

 tympanic has been recognized in Marsupials, Edentates, Insectivores, Carnivores 

 Ungulates and bats. Where it is apparently lacking it may have fused with 

 other bones. 



Premaxilla, usually bearing thecodont incisor teeth, are always 

 present, but are rudimentary in bats and some Insectivores (corre- 

 lated with the reduction of the incisors, although well developed in 

 the toothless Mystacocetes). Each has palatal and ascending 

 processes, but a medial ascending process is rare. The bone bounds 

 the naris in front, except in Echidna where it completely surrounds 

 this opening. The premaxillae of the two sides are usually distinct 

 through Hfe, but in man and Anthropoids they fuse early with the 

 maxillce. They are very long in whales where the nares are far back 

 on the head. The incisive foramina (p. igi) may be between the 

 palatal processes of the two sides or between them and those of the 

 maxillae. 



Each maxilla arises from several centres which soon unite. Each 

 bone almost always bears thecodont teeth, contributes to both 

 facial and palatal surfaces, and is usually elongate, extremely so in 

 whales where the posterior facial part is expanded and overlaps the 

 frontal in Denticetes, or extends beneath it in Mystacocetes. Its 

 palatal process always meets its fellow in the middle line; it contrib- 

 utes to varying extents to the nasal apparatus, the floor of which is 

 largely formed by the palatal processes, while its medial side adjoins 

 the lateral part of the nasal capsule, and when this is absorbed, the 



