WOOD, CRINOID ARMS IN STUDIES OF I'HYLOGENY 9 



Comparison of the Pkeceding Six Species 



A study of the first five of the species just described shows a series of 

 gradations in structural characters which is here interpreted to mean that 

 they form a continuous phylogenetic series in which the tendency of 

 evolution has been from the simpler forms to the more complex. 



Throughout the comparisons which follow, changes in structural fea- 

 tures are assumed to represent stages in development, and the complete 

 series of such changes to express the evolution of the arm as a whole. 



The calyces from Cactocrinus proboscidalis to C. deuticulatus show a 

 progressive increase in the number of features to be considered. The 

 greater number of calyx plates is due to the increase in the number of 

 arms developed, from four in C. proboscidalis to six in C. denticulatus. 

 The elaboration of surface features is expressed in additional carinas and 

 fine nodes covering them in C. denticulatus. 



The arms furnish more conclusive evidence of relationship. In study- 

 ing the arms of crinoids, we have to consider several distinct characters, 

 such as the stage at which the biserial condition is introduced, the form 

 of the arm as expressed in its transverse section, the thickness of the in- 

 dividual plates and the surface features or ornament of the arms. Each 

 of these characters may develop at a different rate of evolution in differ- 

 ent specimens or even in different arms of the same specimen, but in the 

 same phylctic series new features for each will appear in the same order 

 but not, as already stated, necessarily at the same time. For example, we 

 may find in one arm of Cactocrinus denticulatus the median row of nodes 

 well developed on the twenty-fifth plate, while in another arm they are 

 not distinct until the thirty-fifth. The lateral row of nodes may be de- 

 veloped on the sixth plate or not until the fifteenth plate, but the median 

 row never appears before the lateral row. 



Comparing the arms of the five species in detail, we find that Cacto- 

 crinus proboscidalis has a laterally flattened arm with simple transverse 

 ridge. C. baccatus retains the same form and, according to the interpre- 

 tation of the facts here given, passes through the same early stages as its 

 ancestor, C. proboscidalis; that is, first cylindrical, then laterally flattened 

 with a transverse ridge on each plate, but this species goes a step farther 

 in the breaking up of the transverse ridge into a row of nodes. In both 

 these species, the arms taper to a point. The arms of C. platybrachiatus 

 pass through the same early stages as its ancestors, repeating the trans- 

 verse ridged stage of C. proboscidalis, the nodose stage of C. baccatus and 

 adding a feature of its own in the confluence of the nodes at a late stage of 

 ■development. In form, the arms present entirely new features in their 



