NO. 1846. ON CERTAIN ELEUTHEROZOW PELMATOZOA—KIRK. 117 



indeed that they are of two sorts, to a certain extent mntually 

 interdependent. In the first place detachment may be forced upon 

 the organisms by stress of physical conditions under which the animals 

 live. In the second place it may be assumed 'as a character largely 

 benefiting the race. 



A discussion of these factors is of no small interest as showing to 

 what extent the disruption of the column is optional and to what 

 extent it is involuntar}^. There are two possible processes by which 

 detachment may be attained among living Pelmatozoa. In the first 

 place we may have a violent disruption of the column through the 

 agency of some physical force. In the second place we may have 

 voluntary detachment, following partial resorption of the column. 

 Both of these processes have no doubt at times been effective. 



Detacliment by violent disruption. — In order to make valid the 

 hj^'pothesis of violent disruption of the crinoid column one must first 

 have a physical agent competent to cause such a break. The only 

 plausible explanation would be that of severe wave action. The 

 assumption that crinoids may live at depths at which wave activity 

 might act with considerable intensity is not wholly unwarranted. 

 As has been elsewhere cited, Isocrinus has been reported at depths 

 of approximately 20 fathoms. It is said, however, that they live in 

 much shallower water than this even. After heavy storms the shores 

 of certain coasts are said to be strewn with fragments of Isocrinus. 

 Carpenter (1884, p. 289) records that after a gale Isocrinus of all ages 

 and sizes were cast up on the shores at Barbados. This must indi- 

 cate a very shallow water habit for Isocrinus at that locality. Cer- 

 tainly were we to find the modern representatives of the Crinoidea 

 living at such extremely shallow depths as to be destroyed by wave 

 action and cast up on the shore, we may equally well expect the 

 Paleozoic forms to have lived in water shallow enough to subject 

 them to considerable wave violence. 



Involuntary detachment of the crinoids may have been a not 

 uncommon, and consequently an important occurrence in Paleozoic 

 times. As mentioned above, it seems possible that the sea bottom 

 was Cjuite within the range of wave action, at least in cases of violent 

 storms. If this were true we may readily conceive of the crinoids 

 being torn loose from their anchorage and leading an eleutherozoic 

 existence for a time at least, while they were seeking favorable loca- 

 tions for reattachment. The beds of crinoid roots at Waldron may 

 well be an instance of this sort. These were adult specimens and it 

 seems highly improbable that the entire colony would break loose 

 and migrate to another point voluntarily. 



Such involuntary detachment may have been especially common 

 in the case of young forms. It seems reasonable to expect that these 

 may have lived in much shallower water than the adults. The case 



