22^ Ni:\V VOKK STATE MtJSfiUM 



The first vertical column gives the distance of each member of the 

 first triad from the corresponding- member of the second triad, or 

 from that member which came to occupy the next position on the 

 same suture. The second vertical column does the same for the 

 distances between the second triad of b.vs. and the next three b.vs. 

 to appear. The first horizontal line of figures gives the distances 

 for the vertical suture, the second line gives them for the diagonal 

 suture at the left and the last line gives them for the diagonal 

 suture at the right. Interpreted according to the above assumption, 

 the distances indicate that the most rapid growth was after the 

 development of the first triad and between the times of the fixing 

 of b.vs. 3 and 6 on the same suture. This would be during adoles- 

 cence. The marked decrease next the end of the series would 

 indicate that the specimen was practically fully grown or mature. 

 By measuring these distances on a number of different areas and 

 using averages one could plot a curve representing this variation of 

 plate extension in time and the character of such a curve in itself 

 would be good evidence that our assumption was not far wrong. 

 . The b.vs. speak still more clearly of variation of rapidity of plate 

 extension in certain directions. Suppose we ask if the elongate 

 form of the species has been brought about by adding stereom more 

 rapidly to the orad and aborad sutural faces of the plates than to 

 their lateral sutural faces. We may question text figure lo con- 

 cerning this matter. The average distance apart of the b.vs. from 

 2 to 8 (on the right aborad suture of r.post.R) is 6.05 mm; 

 from 3 to 9 (on the left aborad suture of r.ant.R) it is 5.80 mm. 

 Averaging the two we obtain 5.925. On the vertical suture 

 from I to 10 the average is 5.933 mm. The difference here 

 is so remarkably little that it would be unwise to use it as the basis 

 of a declaration that the vertical elongation of the aborad half of 

 a R. was in excess of the lateral extension of one side. We shall 

 soon find, however, that the plates of the two lower circlets give 

 very decided evidence that vertical extension was in marked excess 

 of lateral. We may also note here that r.post.R seems to have 

 added stereom to its right side a little more rapidly than r.ant.R 

 did to its left. This point will be again referred to when we come 

 to discuss the lateral extension of the theca. 



The aborad radial groups. We have so far given particular 

 attention only to one of these groups of b.vs. which developed about 

 the orad angle of each B. These groups are interradial in posi- 



