86 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



of individual variation, as to this supposed primitive character, within the limits of a 

 species. That is to say, in this species, living in the same locality, having the same 

 environment, floating in the same mass, certain individuals matm-ed to represent one 

 stage of larval development, i. e., with infrabasals, and others in another stage, i. e., 

 with basals only. In short, they are the two supposed distinct types, Monocyclica 

 and Dicyclica, occurring in both young and adult of one and the same species. 



Wliy is not the centrale in the so-called "monocyclic" type of 

 Uintacrinus, merely the primitive centrale of the dicyclic type to 

 which the infrabasals have become fused ? Springer, in his memoir 

 on Uintacrinus, argues against the transmutation of one form into 

 the other on various grounds. His arguments may best be brought 

 under several headings, and so discussed. All page references are to 

 his work on Uintacrinus unless otherwise stated. 



On page 25 he says: "The differences presented by forms M and 

 D are not correlated with any other characters. They have no 

 apparent relation to size or maturity of the specimens, and therefore 

 could not have been the result of differences in individual growth." 

 Against this assertion I can offer no more convincing proof than 

 Springer's own subsequent statements: "There is no reason, based 

 upon any characters exhibited by these specimens, for any inference 

 as to one being prior in development rather than the other, unless it 

 is the greater prevalence of Form D among the young individuals." 

 This prevalence of Form D in the young, we find to be very striking 

 indeed, and is of the utmost significance. On page 34 we find the 

 relative proportion of Form D among the young and adult specimens 

 to be as follows: "Among specimens measuring under 25 mm. across 

 the calyx, 75 per cent are dicyclic; while among those of maximum 

 size — 50 mm. and over — the proportion of dicyclic is about 40 per 

 cent." In the face of these figures, he continues (p. 34): "If 

 there were a tendency to develop by individual growth into either 

 form, then we ought to find practically all the adult specimens 

 belonging to that form; and the same thing would be true if the 

 transition took place during the larval stage." 



It is true that we do not have all the adults, or 90 odd per cent of 

 them, "monocyclic," or all the young "dicychc," nor should we 

 expect it. A change from 25 to 60 per cent, which expresses the 

 change in percentage of "monocychc" forms in the young and in the 

 adult stage, is rather startling, however. As a matter of fact, the 

 change is slightly less than that just given, based on Springer's 

 figures given below. On page 86 in a table that recapitulates the 

 data bearing on the number of interbrachial plates and the character 

 of the base, we find that out of a total number of 82 individuals 

 measuring less then 25 mm. in diameter, but 20 are "monocyclic." 

 This gives us a percentage of a trifle over 24 per cent — not 23 as 

 given in this table, where the percentage of "dicyclic" specimens is 

 set down as 77 per cent. Among individuals having a width of 



