1926] Hall: Skull of the Rodent OtospermopldJus gra7nmHrus heecheyi 383 



parts. At the constriction between the two parts of the tympanic 

 seen in E, a small inner third ring of bone, the tympanic annulns, to 

 which the tympanic membrane attaches, is present and appears to be 

 separated by a suture from the tympanic bone. Viewed laterally, there 

 is nothing to indicate the place of attachment of the tympanic annulus 

 from F to P. In Q, however, a clearly defined line (see fig. 15) mark- 

 ing the position of the tympanic annulus is to be seen. Lateral to this 

 line the tympanic is transparent. Mesial to this line the tympanic is 

 often more opaque. In Q the tympanic annulus is firmly fused with, 

 and is perpendicular to, the inner face of the bulla. The line of attach- 

 ment of the tympanic annulus to the inner face of the bulla marks the 

 limit of lateral extension of three bonj^ lamellae which, when developed, 

 divide the tympanic cavity into compartments. In a skull of N age, 

 two lamellae extend from the petrous to the mesial and dorsal walls 

 of the bulla. A third lamella extends in front of the petrous and 

 parallel to the other two. The portion that extends lateral to the 

 petrous bounds the petrotympanic fissure anteriorly. A fourth 

 lamella, thicker than the others and shielding the ear bones, extends 

 from the lateral border of the petrous to the lateral wall of the bulla 

 and bounds the petrotympanic fissure posteriorly. The three septa 

 that extend mesially from the petrous appear first in G. At this time 

 the tympanic has practically completed its growth over the petrosal, 

 the external auditory meatus being but little larger than in older 

 skulls. 



The existence of an element concerning which I have not been able 

 to come to anj^ definite conclusion may be mentioned here. While, 

 apparently, not intimately related to the tympanic, its posterior end 

 lies near the anterior end of the tjmipanic ring. ' This element in C 

 is ossified and lies between the squamosal and alisphenoid. It is much 

 longer than wide and, longitudinally, extends from the posterior union 

 of the alisphenoid and squamosal across the glenoid cavity and to a 

 slight distance in front of the glenoid cavity. I am tempted to believe 

 that this may be a constant element because indications of a differen- 

 tiated region of bone at this same place can be made out in D. D and 

 C, however, are from the same litter and the presence of tliis element 

 might be expected in both animals even if it were merely a Wormian 

 bone. That is, it might be so expected, if we may judge from analogous 

 cases in the genus Marmota. The skulls of a few^ litters of Marmota 

 monax monax that I have examined show, not only the presence of 

 AVormian bones in the same position in skulls of the same litter but a 

 striking correspondence in size as well. These elements were inter- 



