﻿

Table 5 



1 to 4 = 0.5 mm 



4 to 7 = 0.55 mm 



2 to 5= .55 



5 to 8= .40 



3 to 6= .55 





-'^O NEW YORK STATE MUSEUM 



We may now turn again to the questions of variation of rapidity 

 of plate extension in time and in direction. Taking the distances 

 between one b.v. and the next younger on the same suture and 

 arranging as in table 3, we have 



7 to 9 = 0.45 mm 



When we compare these b.vs. with the interrajdial group just 

 studied, we find that they are fewer in number and the distance 

 separating the oldest two on any suture is also less. Were the time 

 intervals regular for b.v. branching, we should have to concede that 

 this group is a younger group and the shorter distance between its 

 oldest members, as shown in table 5, w T ould tend to corroborate 

 this view. We might then be led to believe that the new series was 

 developed from an early branch (perhaps the first) of the older 

 interradial series when the two b.vs. would have been separated by 

 the very short distance across an angle of the young plate. We 

 shall do well, however, neither to accept nor reject the idea if our 

 mind can thus hold judgment long in suspension. We may also 

 note that the younger b.vs. on a suture are nearer together than 

 the older and this again we may interpret as due either to a more 

 rapid formation of b.vs. near maturity or to diminished speed of 

 stereom formation. The latter explanation seems the more probable 

 to the writer. 



We take up again the question of difference in rate of plate ex- 

 tension in direction. From our study of the relative positions of 

 the b.vs. in one of the primary interradial groups we were forced 

 to conclude that the building of stereom on the aborad sutures of 

 the radials was but little, if any, in excess of the building on the 

 lateral sutures of those plates. With equal growth on all sutures 

 of a hexagonal plate there would be no change in plate form. The 

 radials of a mature Palaeocrinus have much the same form as the 

 radials of a very young specimen, at least so far as concerns that 

 portion which lies to the right, left and below the center of the 

 original plate. With the group now under consideration (an 

 aborad radial group) there is evidence for marked difference in 

 rapidity of plate extension in direction. 



In order to present this evidence more clearly and to show its 

 bearings on change in plate form during ontogeny, we have formed 



