386 University of California Pnblicaiions in Zoologi) [Vol. 21 



but just above the transverse base they dwindle to mere threads. In 

 C and D the only portion of any of the auditory ossicles that is ossified 

 is the head of the malleus. In E the malleus and incus are ossified and 

 present much the same appearance as in F. In a skull from the same 

 litter as F the stapes, except for the absence of the stapedial process on 

 its posterior cms and a general slenderness throughout, presents the 

 same features as it does in the adult. The stapedial process is ossified 

 only in animals more than one year old. Increased size of the stapedial 

 process is directly correlated with increased age. In F the incus is 

 slightly more slender than in adults. In F the sylvian apophysis is not 

 firmly attached to the end of the stapedial process, apparently bear- 

 ing much the same relation to the stapedial process that epiphyses do 

 to other bones in the body. At this time the malleus is also lighter 

 throughout than it is in Q. That portion of the head which lies just 

 below the articular surface for the incus is less protruded in F than 

 in Q. At this time, F, the processus cephalicus is less than one-half 

 the length of the manubrium, but in Q the processus cephalicus is 

 more than one-half the length of the manubrium. The lamina between 

 the processus cephalicus and the manubrium is practically as well 

 developed in F as in Q; thus the angle between the processus cephalicus 

 and manubrium appears more obtuse in F than it does in Q. Actual 

 measurements of the angle show, however, that it is the same at both 

 ages; hence the manubrium and processus cephalicus bear the same 

 relations to one another, with respect to direction of growth, in both 

 ages. There is but a slight constriction of the cephalic peduncle at 

 any age and the constriction is proportionately no greater in F than 

 in Q. The manubrium itself is much deeper in Q than in F. In F 

 the manubrium is only about seven-eighths as long as it is in average 

 examples of Q and the distal one-third is greatly curved anteriorly 

 whereas this portion is straight in Q. The broad spatulate end which 

 attaches to the tympanic membrane is twice as large in Q as in F. 

 The processus muscularis is nearly as well deyeloped in F as in Q. 



Mandible.- — It will be noted from figures 28-33 that though the 

 condyle maintains as constant a position relative to the horizontal 

 ramus as does any other portion of the mandible, it assumes a more 

 dorsal position with increased age. The coronoid process becomes 

 more nearly vertical to the horizontal ramus with increasing age. 

 During the same time that the coronoid process is becoming more 

 vertical the angular portion of the mandible extends farther ventral 

 to the long axis of the horizontal ramus. The angular portion of the 

 mandible also enjoys a large amount of growth posteriorly. In B, C , 



