494 HERRICK E. WILSON 



Taking first the quadripartite base, and comparing 2 of the diagrams with 

 8 — one pentangular, the other hexangular — we find that in the latter the 

 posterior basal has doubled in size (7), without materially changing the orien- 

 tation of the plates, or disturbing their general arrangement 



In the tripartite base the change was accomplished in a different way. 

 There x is added to plate c (9 and 10), and plates ab and ed have coalesced, 

 and hold relatively the same position as in 3. 



The bipartite base is probably derived from the tripartite (4), which pre- 

 ceded it in time, and x, which in the latter constituted a part of c, is united 

 with de, and ab with c (n and 12). 



Now taking up 7 and eliminating x, so that the side of plate a rests against 

 the plate e, we obtain 2, and by a similar procedure we are enabled to trans- 

 form 9 into 3. The hexagonal base is thus restored to its primitive pentagonal 

 form without disturbing the orientation of any plate, compound or simple. 



This theory was thought by Wachsmuth and Springer to have 

 been confirmed by an abnormal example of Teleiocrinus umbrosus 

 n which the anal plate was wanting. 



Teleiocrinus umbrosus has normally three equal basals, but in this specimen 

 the basal plate to the left of the anterior ray is reduced to one-half its normal 

 size, leaving the basal disk exactly like that of forms which are normally 

 without the anal plate. 



It is very remarkable that while in all crinoids with an unequally tripartite, 

 monocyclic base, the smaller plate is located to the left of the anterior radial, 

 this plate in the base of the blastoids lies invariably to the right (6). 



In the discussion of the changes from the pentagonal, five-basal 

 form to the hexagonal, four-basal form, it is stated that the enlarge- 

 ment of the posterior basal took place upon the right side of that 

 plate, but no evidence is submitted for this statement. Why 

 could not the enlargement have taken place as well upon the left 

 side, or by symmetrical development upon both sides of the pos- 

 terior basal? The statement that ''the evidence leaves no doubt 

 at what part of the base the extra width was inserted" is not suffi- 

 cient, and it in itself creates a doubt. Again we are told that '"the 

 introduction of the anal into the ring of radials necessitated cor- 

 responding modifications among the basals, as otherwise these 

 plates would lose their inter- radial position." 1 However, in the 

 discussion of the basals in dicyclic crinoids the statement is 

 made that "the introduction of the anal plate into the ring of 



1 Ref. 39, p. 59. 



