58 UIXTACEINUS: ITS STRUCTURE AND RELATIONS. 



known. The mouth might have retained its marginal position, or it might, 

 with the removal of the cause, have returned to its original central position. 

 In favor of the latter view is the fact that the recent Pentacrhiidae, which 

 may be considered to be the successors of the Encrinidje, have no trace of 

 this structure, but have the mouth strictly central, and the gut has but a 

 single coil. In favor of the former is the case of Holocrinus. 



Whatever its origin, the coil of the gut in Actinometra is a structure, as 

 Bather says, which is doubly peculiar, and undoubtedly is a morphological 

 modification of much importance, — although he attaches no importance to 

 it in classification. And there can be no doubt that the same structure 

 existed in Uintacrinus. 



The relations of Uintacrinus will probably be a subject of discussion for 

 some time to come. Neumayr, von Zittel, and P. H. Carpenter have classed 

 it with the Flexibilia. Bather believes its relations to be with the dicyclic 

 Inadunata. Jaekel places it provisionally among the Camerata, along with 

 the dicyclic Rhodocrinidoe. There are difficulties attending each of these 

 theories, and more or less strong reasons can also be adduced for each. 

 Mesozoic Crinoids are so rare, and the gaps between them and their Palceo- 

 zoic predecessors so wide, that comparisons are difficult, and lines of deriva- 

 tion impossible to trace with certainty. All these views, however, will have 

 to be reconsidered in the face of the new and remarkable characters dis- 

 closed by the present material. In the flexibility of the calyx and arms, 

 the possession of an interbrachial system, the large size of the visceral 

 cavity, the distinctness of the axial canal, and the dicyclic base in one 

 form, Uintacrinus certainly exhibits a morphological resemblance to some 

 of the Flexibilia which is of strikins; si";nificance. 



The general absence of true pinnules and of syzygial union in the arms 

 of the Flexibilia Impinnata is undoubtedly an argument against its rela- 

 tion to that branch of this group. But this would not apply to the 

 Pinnata, which go back to the Jui-assic and perhaps to the Trias. It may 

 be true, as remarked by Mr. Bather, that none of the known Flexibilia — 

 by which I suppose he means only the Impinnata — show a predilection 

 for a free mode of life, as do some of the Inadunata. But I would suggest 

 that there was a strong tendency among many of the Flexibilia of that 

 branch for the stem to break off, leaving the top columnal attached to 



