234 ^'E\V YORK STATE MUSEUM 



the b.vs. from the aborad angle of the radial. These are pre- 

 sented in table 8. 



Table 8 

 a=i.^ mm ^=^i.oo mm ^ = 0.90 



d= .8 e= .55 /= .35 



g= .35 (h= .15 ^'= -14 ?) 



Using these to form a table after the pattern of table 6 we have 



Table 9 



a — b (1.30 — 1. 00) =0.30 b — c (i.oo — .9o)=o.To c — d (0.90— 0.80) =0.10 

 d—e ( .80— .55) = .25 e—f ( .55— -35) = -20 /— ^r ( .35— .35) = .00 



This also shows the more rapid vertical extension of the BB. 

 We may again note that the difference between the rates of verti- 

 cal and lateral plate extension is not so great here as in the lower 

 area of this figure. In other words, the vertical extension of the 

 aborad portion of a R was very nearly as rapid as the vertical 

 extension of the orad portion of a B. The portion of b.v.i, so 

 near the center of the vertical suture, shows also that stereom 

 formation was but little more active on the aborad sutural faces 

 of the BB than on their orad. The greatest difference in growth 

 rate was along the vertical sutures and the modification of form 

 was brought about by decreased or inhibited growth aborad 

 rather than by increased growth orad. 



Infrabasals. The absence of the formation of b.vs. at the 

 aborad angles of the BB is correlated with a very marked lack of 

 stereom formation along the common sutures of the IBB. The 

 first b.vs. to extend their arms over the IBB once occupied nearly 

 central positions on the orad shoulders of these plates. Had 

 stereom extension occurred equally at the right and left of these 

 b.vs. they would have retained their central positions, while now 

 they are close to the outer edges of these shoulders. At the time 

 of protrusion of b.v. 2 (see text figure 14) it was about 0.3 milli- 

 meters distant from a vertical line bisecting the plate and con- 

 tinuous with the suture between the IBB below it. Following 

 the external canal down to the suture we find that the opening to 

 the interior was 0.5 millimeters distant from the same line at 

 death. The difference of 0.2 millimeter represents the widening 

 which has taken place on this portion of the B during the last 

 three-quarters of its growth. This very slight divergence of the 

 two longest b.v extensions on a B offer a valuable character for 



