1926] Hall: Slaill of the Rodent Otospermophilus grammurus beccheyi 389 



proportionately greater increase in length of both the post- and pre- 

 alveolar portions of the mandible than of the alveolar portion appears 

 to be a factor involved in the straightening of the mandible. Another 

 factor is the assumption by the whole post-alveolar portion of the 

 mandible of a position lateral to the tooth row. The changes involved 

 in the assumption of this lateral position are better shown by figures 

 34-37 than by any amount of description. Points that will be noted 

 in these same figures in addition to those already indicated are : the 

 reflexing of the angle, the change in position of the condyle, and the 

 development of a more vertical posterior margin of the mandible. It 

 may be noted that the assumption of a position lateral to the mandi- 

 bular tooth row is reflected in the increase of 200 per cent in distance 

 between the glenoid fossae from C to Q while the mandible is increasing 

 only 186 per cent in length. 



Meckel's cartilage Avas noted in D in the course of preparation of 

 the skull but unfortunately its relations to adjacent structures were 

 not recorded. In E the dentary has grown around Meckel 's cartilage, 

 converting the Meckelian sulcus into a tube. Within the lower portion 

 of the posterior half of the horizontal ramus this tube is revealed by 

 several openings over which ossification has not yet occurred. In this 

 region of the mandible, Meckel's cartilage appears to be absorbed. In 

 stage B, represented by figure 33, there is a rather abrupt truncation 

 of the cartilage proper a short distance behind the lower end of the 

 incisor pocket. Between the end of this pocket and the anterior end 

 of Meckel's cartilage there is a cylindrical area of tissue which repre- 

 sents the anterior end of Meckel's cartilage but Avhich is, at this time, 

 of an entirely different nature than the posterior part, having the 

 appearance of being chondrified. Examination of this area in D 

 shows a cavity ; thus it appears that the part of Meckel's cartilage that 

 lies within or along the median side of the mandible is largely if not 

 wholly resorbed. In B Meckel's cartilage is not straight but is bent 

 at three places ; it lies along the extreme ventral margin of the 

 mandible and at this time actually projects slightly below the ossified 

 horizontal ramus (see fig. 33). The posterior end of Meckel's cartilage 

 fits in a socket of the petrosal, beneath the squamosal and slightly above 

 the ventral margin of the latter. If Kingsley's general statement 

 (1917, p. 82) that: "In development the malleus is the posterior oid 

 [italics mine] of Meckel's cartilage" applies in this case — as it prob- 

 ably does — a considerable change has taken place in the relative posi- 

 tions of Meckel's cartilage, the malleus, and the incus. As shown in 



