NO. 1846. ON CERTAIN ELEUTHEROZOIC PELJilATOZOA-EIRK. HI 



lower portion of the body forming a smooth flattened surface, admi- 

 rably adapted to sliding along the bottom. 



Zygocrinus. — In Zygocrinus, from the Carboniferous of England, 

 modifications apparently quite at variance witlj. those to be observed 

 in the case of Eleutherocrinus are to be noted. It is difficult in the 

 extreme without an examination of original material to determine 

 the nature and extent of these differences. It would seem, however, 

 in Zygocrinus, that upon detachment the animal did not rest length- 

 wise upon the bottom, but upon the apical portion of the theca. If 

 this be the case it seems highly probable that the ancestral type was 

 subglobose as to form, and perhaps not unlike Granatocrinus. Motion 

 hi this curious genus seems to have been directed with the aborted 

 ray forward. The genus is certainly a highly specialized and diver- 

 gent type and if it bears movable spmes as described the resemblance 

 to the Echinoidea is considerably enhanced. 



General discussion of Group 11. — In the members of Group II it is to 

 be noted that the greater portion of the column is lost and in certain 

 cases no remnant of it remains subsequent to detacliment. Main- 

 taining an eleutherozoic existence, permanent refixation by cemen- 

 tation has in no case been observed. The loss of the column is even 

 more remarkable than the assumption of a detached existence. 

 The column is a highly differentiated organ and was only acquired 

 after a long period of fixation. That such a highly specialized 

 organ should suddenty be dispensed with is an interesting com- 

 mentary on the way in which animals discard structures when it is 

 to their advantage to do so. The w^ide assumption of an eleuthero- 

 zoic type of life among such extremely variable organisms throughout 

 geologic time further emphasizes the remarkable tendency there has 

 been noted toward the assumption of freedom by the Pelmatozoa. 



Group III. 



The members of Group III as heretofore defined constitute those 

 Crinoidea that do not possess a true jointed column but as a rule 

 are cemented to the bottom b}^ a base of varying composition. Tliis 

 base for purposes of convenience I have styled a peduncle. The 

 bearing that these pedunculate forms have on eleutherozoic conditions 

 as they obtain among the Pelmatozoa may not be immediately 

 obvious. It seems liighly probable, however, that these forms 

 represent an effort on the part of the Crinoidea to assume a detached 

 existence, a tendency that has become but partially effective. Fur- 

 thermore, in one genus at least to which we shall devote most of our 

 attention both an attached and detached existence is maintained, 

 even within a single species. At all events the cause for the assump- 

 tion of such structures as are possessed by the members of tliis group 



