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NEW YORK STATE MUSEUM 



wardly. These are without doubt very near the inner boundary 

 of the plate. There exist in this cross-section more faint and 

 diffuse lines of scattered carbon particles that connect these 

 outer ends of the hydrospires with each other and where the 

 lines can be distinguished they seem to lie on the whole a little 

 nearer the exterior of the plate. This gives one the impression 

 that the hydrospires rested on the internal ridges rather than in 

 the grooves. The fact that the basal hydrospire slits open 

 directly into the grooves seemed to me to negative such a con- 

 clusion. The weaker carbonized lines probably do not mark 

 the inner boundary of the plate but show the outer folds of the 

 membrane forming the hydrospires. If we imagine a series of 

 folds formed as in figure 3 of the text with their outer edges 



~Bhslbidoc 



Fig. 3 A. diagram of the hydrospires of Codaster copied from F. A. Bather's The Echino- 

 derma [part III of A Treatise on Zoology edited by E. Ray Lancaster] p. 83, and a hypo- 

 thetical diagram to show probable manner of formation of the hydrospires in Blastoidocrinus. 



thrown very closely together and adhering in part before stereom 

 formation took place and if we further suppose that stereom was 

 formed more abundantly at the outer folds as in Troostocrinus 

 and Codaster, while at the same time the outside was covered 

 by the thick stereom built above the fold along that wide face 

 of the deltoid supporting the four row^s of plates before men- 

 tioned; then we shall find a condition of things not only as 

 shown in figure 3 but also actually existing in figures k, 1, m, 

 n and o of plate 5 where the ridges are seen to be narrower 

 across their upper edges than the distance across the groove 

 at the same level. 



