372 University of California Puhlicatioiis in Zoology [Vol. 21 



parts of the orbitosphenoids are present in an ossified state. In F 

 ossification is completed. For some time, however, these bones remain 

 very thin as compared with the other bones of the skull. The sutures 

 separating the orbitosphenoids from the other bones within the orbits 

 can be made out in very old specimens. The orbitosphenoids are in 

 contact with the alisphenoids, frontals, palatines, presphenoid, and 

 occasionally the maxillae. The orbitosphenoids extend down over the 

 sides of the presphenoid ; thus they are overlapped by the palatines. 

 The alisphenoids and frontals also overlap the orbitosphenoids though 

 only slightly. 



Alisphenoids. — In A a slight ossification is present only in that 

 portion of each alisphenoid that forms part of the pterygo-alisphenoid 

 ridge. In B ossification has extended up along the anterior margin of 

 the squamosal. In C the ossification has extended on dorsally, bring- 

 ing the frontal into contact with the alisphenoid. At this time it is 

 ossified little if any farther anteriorly than it is in B. After B 

 ossification proceeds gradually to completion in F. Each alisphenoid 

 is in contact with the basisphenoid, a tympanic, a pterygoid, a 

 palatine, an orbito-sphenoid, a frontal, and a squamosal. The orbito- 

 sphenoids, frontals, basisphenoid, and sides of the pterygoids are 

 overlapped by the alisphenoids. The palatines and tympanies meet 

 the alisphenoids rather squarely. The basisphenoid and alisphenoids 

 become firmly united in old individuals. The palatines also become 

 firmly attached to the alisphenoids. Although the unions between 

 the palatines and alisphenoids are firm, sutures can often be made out 

 between the elements in adults. In the base of the alisphenoid two 

 canals extend forward from the foramen ovale. The larger and lower 

 of these two canals extends forward to the foramen lacerum anterius. 

 The smaller of these two canals, posteriorly, begins just above the 

 larger, only a thin partition of bone separating the two, and, 

 anteriorly, opens into the orbit just behind and laterally to the foramen 

 lacerum anterius. Only in the anterior part of its course is it a true 

 canal for, above, it opens into the brain cavity for a short distance 

 after leaving the foramen ovale. In Marmota, where the arrangement 

 of the foramina and their relation to these two canals is essentially the 

 same as in Otospennophilus, both of these canals carry branches of 

 the fifth nerve. Neither carries a blood vessel. Under the heading 

 of "General Morphology of the Rodent Skull," Gregory (1910, p. 329) 

 states that: "The alisphenoid canal (occasionally absent) is appar- 

 ently not the large tunnel leading forward to the foramen lacerum 



