REPORT OF TliE DIRECTOR iQtO 227 



pass from the primitive arrangement in which its b.vs. simply 

 occupied the hnes of plate boundaries to the more complex arrange- 

 ment by which its b.vs. came to occupy the area of plate surfaces. 

 It is probable that in Palaeocrinus the development of T-shaped b.vs. 

 was not so simple as the processes above outlined. The youngest 

 b.vs. found in the type are already a little distance from the corners 

 and appear as small low mounds that sometimes show more on one 

 side than the other. It looks as if free or uncovered external b.vs. 

 had already been suppressed and that the extension to the surface 

 did not break through the epidermis, but lifted it as a mound which 

 became elongated with the growth of the plate and under which the 

 b.vs. extended their arms. As several layers of tissue are involved 

 in these body wall extensions, the outer layer or the next layer under 

 this might form porous stereom and the deeper layers form the 

 walls of a contractile tube free to move inside of its rigid but porous 

 covering. Evidence of additional (abnormal) forking is shown in 

 plate 5, figure 4. Both of the epithecal canals below the longest are 

 distinctly double near the suture but are single in their earlier por- 

 tions. At least three b.vs. here came to empty into a single sutural 

 canal. One arm of the first fork evidently tried to repeat the 

 process. 



The b.vs. as indexes of plate growth. Could we determine the 

 rate or regularity of b.v. development we could use the position of 

 these structures as indexes of the relative rapidity of plate growth 

 during different ontogenic stages. If the new b.vs. appeared at 

 regular time intervals their distance apart on any suture would be 

 in strict proportion to the rate of plate extension. Text figure 11 

 may be made to offer an illustration. The measurement of position 

 of the b.vs., in part recorded in the figure, is as follows : 









Table 



2 









1 



21 .5 mm 



4 



15.8 mm 



7 



9.7 mm 



10 



3.7 mm 



2 



19. 1 



5 



12.7 



8 



7.0 



11 



2.5 



3 



17. 1 



6 



10.5 



9 



5-5 



12 



I.O 



From the measurements of position recorded in table 2 we have 

 deduced the following table of distances between one b.v. and the 

 next younger on the same suture. 



Table 3 



1 to 4 = 5.7 mm 4 to 7 = 6.1 mm 7 to 10 = 6.0 mm 



2 to 5 = 6.4 5 to 8 = 5.7 8 to 11=4.5 



3 to 6 = 6.6 6 to 9 = 5.0 9 to 12 = 4.5 



8 



