NO. 1840. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 63 



exception. Here the cirri are preserved even to the most tenuous 

 extremities. 



Crinoid c^o^\^ls, with their comparatively weak articulations, are 

 found in the utmost perfection. These, moreover, lay upon the sea 

 bottom and were not already covered. Comparing the two extremi- 

 ties of a crinoid, all the chances for preservation lie with the roots. 

 The relative proportions in which the two are found, however, would 

 tend to prove quite the opposite. The crowns and calices far out- 

 number the roots. The only logical deduction from these facts is 

 that there were in reality more crowns than roots — in other words, that 

 a very great number of the Paleozoic Crinoidea, like their modern 

 relatives, led a semifree existence. 'At any rate, we may assume 

 that they had no highly specialized organs for purposes of permanent 

 fixation. 



One must constantly bear in mind that universal freedom is not 

 predicated for any given time or for any given group. It was con- 

 stantly assumed in a number of sporadic lines, but these may be 

 held as exceptions to the general rule. In some species and genera, 

 no doubt, an eleutherozoic existence was of very rare occurrence. 

 Certain specialized roots bear witness to this fact by their great 

 abundance. In the case of the easily recognized " Aspidocrinus," for 

 example, which is the basal expansion of some apparently adult 

 crinoid column, hundreds of specimens may be collected in certain 

 beds. No crinoid crowTis are found associated, having doubtless 

 decayed and been reduced to their constituent elements. Such an 

 instance illustrates the proper proportion one would think should 

 obtain between crowns and roots, were there originally an equal 

 number of each, and were the bases of attachment practically inde- 

 structible under normal conditions. The proportion should be more 

 nearly equalized in cases where the root^ are composed of cirri, 

 perhaps, for here there is a greater chance for the roots to be 

 destro3^ed. 



The evidence as afforded by available facts relative to the material 

 discussed above seems to warrant the drawing of two general con- 

 clusions. In the first place we are fairly safe in assuming a fixation 

 of the crinoids in their early stages, which was frequently, perhaps 

 universally, followed by a period of detachment and freedom. In 

 the second place it seems highly probable that there was a widespread 

 lack of fixation among the mature crinoids. Given freedom and 

 more or less ability to move, we may easily see that the crinoids 

 could largely determine and control their movements, and conse- 

 quently their environment. 



As suggested above, it seems highly probable that these periods of 

 postlarval freedom bear largely on the distribution and segregation 

 of the crinoid elements in the various faunas and have much to do 



