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NEW YORK STATE MUSEUM 



extended. If now these plates have so grown as to give their 

 common sutures equal extensions, it will follow that the distance 

 of any b.v. from the point of meeting of these three sutures is 

 proportional to its age, provided we do not pass the middle of, 

 the suture or the longest exothecal canal which crosses it. Let 

 us apply this rule to the region around the orad angle of r.post.B. 

 The primary interradial groups of branchial vesicles. On the 

 photographs used for figures 3 and 4 of plate 5 was measured the 

 distance of the middle of each sutural canal from the apex of the 

 orad angle of r.post.B. The average of each pair of measure- 

 ments was taken as the distance of the b.v. in question. These 

 positions are now indicated by dots on text figure 10 and the 



distances there recorded. The 

 b.v. 21.5 mm out is by our 

 rule made the oldest and is 

 marked no. 1. The next 

 smaller distance is 19. 1 mm 

 and this b.v. has been 

 marked no. 2. These larger 

 numbers thus indicate the 

 order of succession and it 

 turns out to be a regular 

 spiral with a counter-clock- 

 wise rotation. The very reg- 

 ular sequence and rotation of 

 the twelve b.vs. constituting 

 this triangular area (but ten 

 have been numbered in the 

 ... , .. , ., .. . . . _ , figure) is remarkable enough 



Fig. 10 View of a portion of the holotype of Palae- o / «-» 



ocrinus striatus Billings. From a photomicro- £q merit further COnsidera- 

 graph, xio. The spiral is drawn to show the order 

 of development of the b.vs. tion. 



Suppose that during the nepionic stage a b.v. was developed at 

 the point of contact of these three plates and that soon after its 

 exit it had budded a new b.v. aborad, which also sought exit at 

 the same point. No. 1 would be thrust orad and reach in this 

 direction to exercise its function. The growing points of r.post.R 

 and r.post.B would push by and finally surround it, thus giving 

 it a position not at the corner but on the suture. A third b.v. 

 seeking exit also at the plate corners might force no. 2 to the 

 left and would itself, in seeking freedom for its function, pass to 

 the right and thus become inclosed between the growing points 



