INTERRELATIONSHIPS 6j 



support this new plate. Here we have the beginning of the armlike anal series. 

 If the process is pushed a little further, so that the depressed lateral margins 

 of the large anal plate become replaced by perisome, we shall have a complete 

 Taxocrinus anal side, and our Lecanocrinus will have been transformed into a 

 Gnorimocrinus. It is now very curious that the tendency is actually in this 

 direction in regard to those other characters on which the two families have 

 been separated ; for along with these changes in the anal plate there appear 

 marked depressions between the rays and their divisions, which become 

 rounded, with strong tendency to divergence in the upper portions, and to long 

 and delicate arms. I have described this as a modification from Lecanocrinus 

 toward Gnorimocrinus. Of course, we do not know which way it really was, 

 and it is cmite possible that the process of modification was in the opposite 

 direction. 



Which of these two plans was the primitive one we cannot determine from 

 their paleontological history, as they both doubtless run far back into the 

 obscurity of pre-Ordovician epochs. The earliest known species of undoubted 

 Flexibilia type — Protaxocrinus elegans and P. laevis from the Trenton lime- 

 stone — have the Taxocrinus anal side; and, as I shall show later on, the type 

 was clearly derived from the Inadunata. The opposite anal plan, which is that 

 of the Camerata, is represented in Cleiocrinus from the same horizon, which 

 is perhaps a transition form with some flexible and some cystid characteristics. 

 There is, however, to be considered the cmestion whether the morphological 

 differences which gave rise to this structure have not continued to the present 

 time in some of the Articulata; and we may, in that connection, venture an 

 opinion as to their probable origin. 



Many of the modifications in the external form of the calyx were un- 

 doubtedly 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 not be greatly affected by its position, except in the production of 

 bilateral symmetry by the tubelike structure. If it issued ventrally, and from 

 the center of the disk at an early ontogenetic stage, the skeleton would not be 

 influenced by it at all, but should have perfect pentamerous symmetry. Be- 

 tween these two extremes there would be a wide range of variation in the out- 

 ward form and arrangement of the anal side. And we may suppose that as 

 the anus shifted from a very low, lateral position toward the margin of the 

 disk, a series of tube plates would appear and gradually increase in number 

 until the tube should become a distinct structure, or should lose its distinctive 

 character by becoming suturally united, to the rays with or without the inter- 

 vention of other plates, like those of the other interbrachial areas. By the 

 time the anus had moved well into the central part of the disk, where lateral 

 support was unnecessary, all plates of the anal series would be eliminated from 



