502 



FRANK SPRINGER 



scribed this as a modification from Lecanocrinus toward Gnorimo- 

 crinus. Of course, we do not know which way it really was, and it 

 is quite possible that the process of modification was in the oppo- 

 site direction. 



Which of these two plans was the primitive one we cannot deter- 

 mine from their paleontological history, as they both doubtless run 

 far back into the obscurity of pre-Silurian epochs. The earliest 

 known species of undoubted Flexibilia type — Taxocrinus elegans and 

 T. laevis from the Trenton Limestone^ — I believe to have the Taxo- 

 crinus anal side f and, on the other hand, Cleiocrinus, from the same 

 horizon, which is at least a transition form with some flexible char- 

 acteristics, has the opposite anal plan. There is, however, to be 

 considered the question whether the morphological differences which 

 gave rise to this structure have not continued to the present time in 

 the Flexibilia Pinnata; and we may, in that connection, venture an 

 opinion as to their probable origin. 



The modifications in the external form of the calyx were 

 undoubtedly due to differences in the position of the anus. If the gut 

 issued from the visceral mass laterally, and remained there, the 

 growing skeleton would be affected by its position, even to the extent 

 of simulating its outline; and the lower down it issued, the more 

 pronounced would be the separation of the rays by the tubelike struc- 

 ture. If it issued ventrally, and from the center of the disk, the 

 skeleton would not be influenced by it at all, but should have perfect 

 pentamerous symmetry. Between these two extremes we should 

 have a wide range of variation in the outward form and arrangement 

 of the anal side. And we may suppose that, as it shifted from a very 

 low, lateral position, toward the margin of the disk, the tubelike row 

 of plates would gradually disappear, or lose its identity by becoming 

 incorporated in plates suturally united to the rays, hke those of the 

 regular interbrachial areas. By the time it had shifted well into the 

 central part of the disk, all anal plates would be ehminated from the 

 skeleton. Or the shifting might have taken place in the opposite 

 direction, and the order of the changes in the skeleton have been 

 reversed accordingly; although the first is more in accordance with 



I This supposition is confirmed by the discovery of specimens since the above 

 was written. 



