178 THE SKULL. [CHAP. 



more often small and rudimentary. The face is usually 

 short and broad, in some (as Mormops] bent upwards on the 

 cranium in a remarkable manner, so that the plane of the 

 palate is nearly perpendicular to the basicranial axis. The 

 premaxillae are generally small, sometimes not meeting in 

 the middle line, and sometimes (as in Megaderma) altogether 

 wanting. The tympanics are annular, not ankylosed to the 

 surrounding bones, nor prolonged into a bony canal ex- 

 ternally, though often developing a partial bulla on their 

 inner side. 



The mandible has a distinct angular process. 



Order RODENTIA. In the Rodentia the cerebral cavity is 

 generally elongated, depressed, somewhat broad behind and 

 narrow anteriorly. The occipital plane is more or less 

 vertical ; the cerebellar fossa altogether behind the cerebral, 

 and the tentorial plane, or division between these fossae 

 approaching the vertical. The anterior part of the cerebral 

 fossa is contracted ; the olfactory fossa is of moderate size, 

 and situated directly in front of the cerebral. 



The nasal cavities are very large, and both sets of 

 turbinals well developed, including an upper or nasoturbinal 

 lamella. The olfactory chambers attain their maximum of 

 development in some of the Porcupines (Hystrix), where 

 nearly all the bones of the upper part of the cranium are 

 expanded by great air sinuses formed within their walls. In 

 the Hare, and some others, the two optic foramina in the 

 orbitosphenoids are confluent ; and in consequence, in the 

 dried skull, there is a direct aperture of communication 

 between the orbits above the craniofacial axis. 



The supraoccipital is more or less vertical, and does 

 not extend far on to the upper surface of the cranium. 

 There is often a distinct interparietal. There are generally 



