374 



ANATOxAIY OF VERTEBEATES. 



The cranial air-cells continued from the nasal and tympanic 

 cavities reach the occiput. The tympanum is divided by a hori- 

 zontal partition into an upper and lower chamber, intercommuni- 

 cating posteriorly above the membrana tympani, which is situated 

 in the lower division, where the meatus auditorius externus ter- 

 minates in a narrow oblique slit. The extraordinary extent of 

 the air-sinuses surrounding the fore part of the cranial cavity and 

 developed in the orbitosphenoids, alisphenoids, squamosals, and 

 frontals, with the radiating bony septa of those sinuses, are pecu- 

 liarities of the Porcupine. 



In an almost full-grown Beaver, fig. 240, the elements of the 

 occipital bone are still unanchylosed ; the lower tliird of each 

 condyle is formed by the basioccipital, the under surface of which 

 presents a large and deep excavation. The upper part of the 

 2^Q foramen magnum is com- 



pleted by the broad super- 

 occipital. The mastoid is 

 larger than in the Porcu- 

 pines, and articulates ante- 

 riorly with both the parietal 

 and squamosal ; it is anchy- 

 losed to the petrosal. There 

 is a perforation in the suture 

 between the superoccipital 

 and mastoid. The inter- 

 parietal is large, and wholly 

 upon the upper surfiice of 

 the cranium. The squamosal is perforated behind and below the 

 root of the zygoma. The frontals are small and almost flat above. 

 The nasal bones extend further back than the premaxillaries, in 

 the European Beaver, beyond the transverse line which extends 

 between the antorbital tuberosities. The anterior root of the 

 zygoma formed by the maxillary is a simple plate which appears 

 to be imperforate, the orifice of the slender antorbital canal being 

 concealed by a vertical ridge of the maxillary, which inclines 

 forward over the maxillo-premaxillary suture. 



The epencephalic compartment is lower and broader than in 

 the Porcupine. The cerebellar fossa of the petrosal is larger 

 and deeper. The upper compartment of the tympanum is much 

 less. The length and direction of the auditory meatus is shown, 

 fig. 240, o : it changes its form into a transverse fissure, as it 

 approaches the membrana tympani, the plane of which is almost 

 parallel Avith that of the meatus itself. There are no nasal air- 



Skull of the Beaver (Castor Fiber). 



