SKELETON OF RODENTIA. 



375 



241 



Skull of the Ondatra {Fiber zibcticus). 



sinuses in the cranial bones of this aquatic Rodent, and their 

 texture is denser than in most of the order. The sella turcica 

 is extremely shallow, and without clinoid processes : the middle 

 of the basioccipital is reduced by the excavation on its under 

 surface to extreme thinness. A small vacuity in the basisphe- 

 noid communicates with the cranial cavity close to the ' fissura 

 lacera anterior.' The presphenoid is perforated transversely. The 

 rhinencephalic fossa is well marked. The anterior end of the 

 vomer articulates Avith both the maxillary and premaxillary bones, 

 as in the Rat. 



In the skull of the Ondatra or Musk Yole {Fiber zibeticus, 

 fig. 241 ), the basioccipital 

 is not excavated, as in 

 the Beaver, but there is 

 the same perforation be- 

 tween the mastoid and 

 superoccipital, and a large 

 vacuity in the posterior 

 process of the squamosal 

 communicating directly 

 A\ith the cranial cavity. 

 The squamosal is unusu- 

 ally expanded above the zygomatic process, and articulates 

 largely mth both frontal and parietal. The zygomatic process of 

 the maxillary reaches almost to that of the squamosal, and sup- 

 ports a great part of the malar bone. The antorbital foramen, v, 

 is larger than in the Beaver, but is bounded externally, as in it, 

 by a nearly vertical ridge of the maxillary. The interorbital 

 septum is perforated behind, beneath the orbitosphenoid. There 

 is no distinct lacrymal bone ; but the turbinal bones appear at 

 the fore part of the orbit between the two processes of the 

 maxillary which join the frontal, and above the aperture com- 

 municating with the nasal cavity. The anterior j^art of the 

 maxillary, in front of the antorbital foramen, is swollen, and 

 forms a curved canal commencing by an oblique aperture su- 

 periorly, and descending outward and back^vard round the 

 socket of the superior incisor to terminate in the nasal meatus : 

 this part may probably protect the lacrymal sac and duct. The 

 interparietal is a transversely quadrate bone. The sagittal suture 

 is retained, and the upper surface of the parietal is smooth, and 

 nearly flat : the temporal ridges meet and develope a crest upon 

 the narrow frontals, obliterating the frontal suture. The back 

 part of each ramus of the lower jaw is trident-shaped from the 



