X.] CHIROPTERA. 155 



basihyal, with which the elongated, laterally compressed and 

 curved thyrohyals are commonly ankylosed. The anterior 

 cornu contains three slender ossifications of nearly equal 

 lengih. 



The Insectivorous Bats generally have the skull shorter 

 and broader than the Pteropi. The cranial cavity in many 

 species is almost globular, with thin smooth walls, though 

 sometimes sagittal and occipital crests are developed. The 

 occipital foramen is very large. The zygoma is slender. 

 Postorbital processes are sometimes well developed, but 

 more often small and rudimentary. The face is usually 

 short and broad, in some (as Monnops) bent upwards on the 

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

 palate is nearly perpendicular to the basicranial axis. The 

 premaxilke are generally small, sometimes not meeting in 

 the middle line, and sometimes (as in Alegaderjiid) altogedier 

 wanting. The tympanies are annular, not ankylosed to the 

 surrounding bones, nor prolonged into a bony canal ex- 

 ternaU}^, though often developing a partial bulla on their 

 inner side. 



The mandible has a distinct angular process. 



Order Rodentla, — 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 fossje, 

 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 



