NO. 1846. ON CERTAIN ELEUTHEROZOTG PELMATOZOA—KIRK. 91 



possessor of two columns ? Again, if, as I believe, the centrale merely 

 represents a plate to fill up a gap — an entirely new element — ^w^hy 

 should two plates have been necessary to fill up one small opening ? 

 A glance at this figure will show clearly that such an explanation is 

 quite out of the question. There is but one possible explanation of 

 this base and that is a fusion of the plates. If there is fusion here 

 we may expect it elsewhere. Under the discussion of the third 

 hypothesis I shall deal with this coalesence of the plates in some detail. 

 I msh at present merely to point out that we have every reason to 

 believe that fusion among the plates did take place. 



If we attempt to explain figures 7, 6, and 5 as cases where one, 

 three, or four infrabasals only have appeared, the question involved 

 is a far greater one than the mere explanation of the apparent vagaries 

 of Uintacrinus socialis. It may be stated almost as a law that 

 each of the circlets, composed either of basals or infrabasals, acts as a 

 unit in evolutionary changes; that is to say, if, for instance, the 

 tendency in evolution is toward an elimination of the infrabasals, 

 all are equally affected. The plates finally become of so little impor- 

 tance as to be practically a negligible quantity, thus forming a pseudo- 

 monocyclic type. Again, one or more of the plates of a circlet may 

 not drop out, leaving the remainder to shift for themselves. 

 Naturally certain plates of a circlet may be modified and become 

 larger than the others, as is well shown by the basals of Ehuthero- 

 crinus, all five plates being represented. This rule, it seems to me, 

 holds absolutely in regard to the presence or absence of plates. If 

 one plate of a circlet be present, the other four are present, except 

 in purely abnormal individuals. So here in Uintacrinus I do not 

 believe it possible for one, three, or four infrabasals to be present 

 and the others absent. Hence, any apparent reduction in number 

 must necessarily be due to fusion. Of course, one may claim that 

 all these specimens cited are abnormal, but as long as the structures 

 observed can be explained as the result of a perfectly normal process, 

 and, indeed, are exactly what we should expect to fuid, it seems rather 

 unnecessary. As before stated, I believe that the change from 

 Form D to Form M has been brought about by a fusion of the plates. 

 At the same time more or less resorption took place, so these two 

 processes wiU be discussed together. 



On page 32 and following, Springer raises the following objections 

 to the fusion theory other than those already discussed. In the first 

 place, he has never seen a specimen in which the coalescence of infra- 

 basals and centrale would result in the formation of a plate having the 

 outline of the centrale actually found in Form M. The centrale in 

 Form M is pentagonal, whereas the resultant plate he thinks should 

 be stellate. In the following passage which I quote from page 32, 

 the qualifying phrases, which I shall here j^lace in italics, somewhat 



