WOOD, VRINOID ARMS IN STUDIES OF PHYLOGENY 7 



that position. This node continues to increase in size at the same time 

 that the lateral node diminishes. Later, as the arm approaches its dorso- 

 ventrally flattened form, the lateral node disappears, the former median 

 node is elongated into a projecting spine and the newly introduced median 

 node is now a prominent feature of the ornament. The remainder of the 

 arm is incurved and buried in the matrix, hence the character of the 

 latest formed plates is unknown. 



The column is not preserved on any of the specimens at hand. 



HoEizoN AND ix)CAi.iTY : IjOwer Burlington limestone, Burliug1:ou, Iowa. No. 

 527, Museum of Comparative Zoology collection. 



Cactocrinus denticulatus Wachsmuth and Springer 



riate IV, figs. 1, la, 2, 2a, 2b 



1897. Cactocrinus denticulatus Wachsmuth and Springer, North American 

 Crinoidea Camerata, p. 606, pi. 57, figs. Tk;. 5h. 



Caciocrinus- denticulatus is closely related to C. reticuJatus. The ar- 

 rangement of the calyx plates is the same, except that in C. denticulatus 

 six arms to the ray form a constant feature necessitating a greater num- 

 ber of plates for their support. The ornament is somewhat more elabo- 

 rate from the fact that small nodes are present on the costae of the upper 

 part of the calyx, and these sometimes extend as irregularly placed nodes 

 over the base of the arms. 



The arms pass through the same series of changes in form as those 

 described for Cactocrinus reticidatus, except that the early condition with 

 elongat-e plates is not present, and the successive changes up to the dorso- 

 ventral flattening appear at a relatively earlier period in the development 

 of the arm than in the preceding species. The latest exposed plates of 

 the arm have a more extreme form than the corresponding plates of C. 

 reticuJatus. They are more strongly flattened dorso-ventrally, have longer 

 spines, and the nodes are so high and pointed that, in some specimens, 

 they might almost be called spines. Still further differences appear in 

 the lateral spines, which are often alternately longer and shorter, and 

 when this is the case, the nodes also alternate in size, the larger node 

 occupying the plate with the shorter spine. This alternation in size of the 

 spines and nodes is not perfectly regular, but it is a pronounced tendency 

 which manifests itself to some extent on all the specimens studied. The 

 nodes are not, as in tlie preceding species, close to the median line but 

 have receded to some distance from it. 



The column is missing from all the specimens studied. 



HoHizoN AND LOCALITY : Lower Burlington limestone, Burlington, Iowa. No. 

 r»34, Museum of Comparative Zo()logj' collection. 



