98 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



Springer seems to favor descent from the Flexibilia. On page 58 

 he records the tendency among certain Flexibiha to separate from 

 the stem just below the proximale. If the proximale be true to its 

 definition it will always remain, fusing mth the proximal elements 

 of the cup. In Uintacrinus, as shown above, this condition can not 

 obtain. As long then as only those forms having a persistent proxi- 

 male are to be referred to the Flexibilia, so long must Uintacrinus 

 seek relationship elsewhere. The Camerata as the ancestral stock, 

 as suggested by Jaekel, is a proposition scarcely to be considered 

 seriously. By a process of elimination, then, we have narrowed the 

 field down to the Inadunata, in this agreeing with Bather. To 

 attempt to fix the line of descent any more definitely is, I think, at 

 the present state of our knowledge, somewhat premature. 



The stalked Inadunata ancestor should, I think, unlike Dadocrinus 

 chosen by Bather, be a distinctly dicyclic form, not pseudomonocyclic. 

 The apparent elimination of the infrabasals in some specimens of 

 Uintacrinus is a feature acquired very late in the evolution of the 

 genus. In this connection the possibility of the infrabasals of a 

 pseudomonocyclic form regaining their former importance as ele- 

 ments of the dorsal cup, upon the loss of the stem, is a consideration 

 not to be lost sight of, as pointed out above. In such a case, how- 

 ever, it is very difficult to see why the tendency shown by Uintacrinus 

 socialis toward a coalescence of centrale and infrabasals, which is 

 merely an expression of the general trend of all crinoids toward a 

 more or less complete elimination of the plates of the proximal circlet 

 as distinct elements, should show itself so late in the developmental 

 series. One would think that in their almost completely atrophied 

 condition in the pseudomonocyclic form, the infrabasals would at 

 once fuse with the new element, the centrale, rather than remain as 

 minute independent plates. Whatever be the genetic line that 

 evolved Uintacrinus it is probable that we shall ultimately come to 

 a type that will serve as a common ancestor for both Dadocrinus and 

 Uintacrinus. This type again may serve equally well for many of 

 the phyletic lines that flourish in Mesozoic and later times. 



Springer (1901, p. 55) objects strongly to the placing of Uintacrinus 

 in the Inadunata. In speaking of the relationsliip of Uintacrinus to 

 Dadocrinus he says: "In the essentials of structure upon which the 

 great groups of Camerata, Inadunata, and Flexibilia have been dis- 

 tinguished, Uintacrinus seems to me far more widely separated from 

 this group than from the others." In regard to its positive rela- 

 tionsliips he says: "It must be evident that the line of derivation of 

 Uintacrinus will have to be considered in connection with the Coma- 

 tulse. Whatever its ancestry may have been, it is quite plain that one 

 of its near relatives was Actinometra." It is mainly on the evidence 

 of similarity of structure as induced by the mutual possession of 



