232 NEW YORK STATE :MUSEUM 



jx)int. If now we construct a table after the manner of table 6, but 

 using text figure 13 for the basis, we shall have the result here given 

 as table 7. A comparison of the two tables will speak eloquently 

 for our contention. • 



Table 7 



1—2 (3.6— 2.4) = 1.2 2— 3 (2.4— 2.1) =0.3 3— 4 (2.1— 2.4) = — . 3 

 4—5 (2.4—1.5)== .9 5—6 (1.5—1.2)= .3 6— 7 (1.2— 1.2) = — .0 



7_8 (1.2— .6)= .6 8—9 ( .6— .3)= 3 



We shall find that the ratio between lateral and vertical extension 

 varied during development. Between the appearance of b.vs. i and 

 4 the vertical suture of this group increased 0.5 mm in length. 

 Between the appearance of 2 and 5 the left diagonal suture increased 

 0.55 mm. Lateral extension appears to have been slightly in excess 

 of vertical at this time. They soon become equal. Between the 

 appearance of 5 and 8 the lateral expansion was only 0.4 mm. At 

 this period of growth the ratio between vertical and lateral ex- 

 tension seems to have reached the ratio of 1 1 to 8. 



A study of the basals as presented in the cup analysis and in 

 the photograph will show that this ratio is not far out of the way, 

 but it must be remembered that it is not for the whole B but for 

 the lower part of it and that the ratio is one that changes all along 

 the edge of the vertical suture, the most rapid addition being at 

 the top of this suture where it practically equals the rate of addi- 

 tion to its orad sutures and the least rate being at the bottom or 

 aborad end of the same vertical suture. This growth ratio clearly 

 indicates that very young plates were no longer than wide or, in 

 other words, that they were practically symmetrical and so sug- 

 gestive of cystidean plates. \\'ith the aid of this ontogenic evidence 

 we have attempted to present the probable outlines of two young 

 or primitive plates in text figure 9. 



It is interesting to note that this increase of vertical plate exten- 

 sion had carried b.v. 7 so far from the apex of the IB at the time 

 of the emergence of b.v. 9 as to oiTer the latter as much freedom, 

 apparently, on the suture occupied by b.v. 7 as on the suture occu- 

 pied by b.v. 6. The irregular position of b.v. 9 in figure 12 is thus 

 seen to be in harmony with our suggested physical reason for 

 rotation and to rather strengthen that hypothesis, for heredity 

 should have placed this b.v. next to no. 6. The position occupied 

 l)y b.v. in this group is not normal for all of the other IBB possess 

 a third b.v. on their right-liand up])er sutural faces with the pos- 

 sible, though not probable, exception of 1. post. IB. The U]:)per cih^yQ 

 of this plate has been so badly damaged that an accurate count is 



